Before writing a replacement merge strategy for recursive, I decided to
first cleanup the merge API -- streamlining merge-recursive.h and making
it more readable.  This includes some minor fixes found along the way.

Changes since v3 (full range-diff below):
  * Addressed feedback from Junio and SZEDER:
    * [Patch 4] Avoided looping over commit_list in determining
                ancestor_name
    * [Patch 12] s/inmemory/in_core/ in new cache-tree function name
    * [Patch 18] restored the '>= 0' comparision
    * [Patch 21] Use FREE_AND_NULL
    * [Patch 23] Use range checks for show_rename_progress (don't use
                 a bitfield), and don't compare an unsigned using >= 0.

Notes:
  * Patches 7, 9, 13, 14, and 15 only changed in the range-diff due to
    context changes from other patches

Derrick Stolee (1):
  merge-recursive: introduce an enum for detect_directory_renames values

Elijah Newren (23):
  merge-recursive: be consistent with assert
  checkout: provide better conflict hunk description with detached HEAD
  merge-recursive: enforce opt->ancestor != NULL when calling
    merge_trees()
  merge-recursive: provide a better label for diff3 common ancestor
  merge-recursive: future-proof update_file_flags() against memory leaks
  merge-recursive: remove another implicit dependency on the_repository
  Ensure index matches head before invoking merge machinery, round N
  merge-recursive: exit early if index != head
  merge-recursive: remove useless parameter in merge_trees()
  merge-recursive: don't force external callers to do our logging
  cache-tree: share code between functions writing an index as a tree
  merge-recursive: fix some overly long lines
  merge-recursive: use common name for ancestors/common/base_list
  merge-recursive: rename 'mrtree' to 'result_tree', for clarity
  merge-recursive: rename merge_options argument to opt in header
  merge-recursive: move some definitions around to clean up the header
  merge-recursive: consolidate unnecessary fields in merge_options
  merge-recursive: comment and reorder the merge_options fields
  merge-recursive: avoid losing output and leaking memory holding that
    output
  merge-recursive: split internal fields into a separate struct
  merge-recursive: rename MERGE_RECURSIVE_* to MERGE_VARIANT_*
  merge-recursive: add sanity checks for relevant merge_options
  merge-recursive: alphabetize include list

 builtin/am.c                      |   2 +-
 builtin/checkout.c                |  14 +-
 builtin/merge-recursive.c         |   4 +
 builtin/stash.c                   |   2 +
 cache-tree.c                      |  85 +++--
 cache-tree.h                      |   3 +-
 merge-recursive.c                 | 566 ++++++++++++++++++------------
 merge-recursive.h                 | 164 +++++----
 sequencer.c                       |   5 +-
 t/t3030-merge-recursive.sh        |   9 +-
 t/t6036-recursive-corner-cases.sh |   8 +-
 t/t6047-diff3-conflict-markers.sh | 189 ++++++++++
 12 files changed, 717 insertions(+), 334 deletions(-)
 create mode 100755 t/t6047-diff3-conflict-markers.sh

Range-diff:
 1:  21f1e04dc9 =  1:  7f99c2401e merge-recursive: be consistent with assert
 2:  ac24702773 =  2:  b305348f45 checkout: provide better conflict hunk 
description with detached HEAD
 3:  fd14ed9490 =  3:  5b9e32a356 merge-recursive: enforce opt->ancestor != 
NULL when calling merge_trees()
 4:  540a1d17d7 !  4:  566a55cf76 merge-recursive: provide a better label for 
diff3 common ancestor
    @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
        struct commit *merged_common_ancestors;
        struct tree *mrtree;
        int clean;
    -+  int num_merge_bases;
    ++  const char *ancestor_name;
     +  struct strbuf merge_base_abbrev = STRBUF_INIT;
     +
     +  if (!opt->call_depth)
    @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
        if (show(opt, 4)) {
                output(opt, 4, _("Merging:"));
     @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
    -                   output_commit_title(opt, iter->item);
    + 
    +           tree = lookup_tree(opt->repo, opt->repo->hash_algo->empty_tree);
    +           merged_common_ancestors = make_virtual_commit(opt->repo, tree, 
"ancestor");
    ++          ancestor_name = "empty tree";
    ++  } else if (ca) {
    ++          ancestor_name = "merged common ancestors";
    ++  } else {
    ++          strbuf_add_unique_abbrev(&merge_base_abbrev,
    ++                                   &merged_common_ancestors->object.oid,
    ++                                   DEFAULT_ABBREV);
    ++          ancestor_name = merge_base_abbrev.buf;
        }
      
    -+  num_merge_bases = commit_list_count(ca);
    -   merged_common_ancestors = pop_commit(&ca);
    -   if (merged_common_ancestors == NULL) {
    -           /* if there is no common ancestor, use an empty tree */
    +   for (iter = ca; iter; iter = iter->next) {
     @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
        if (!opt->call_depth)
                repo_read_index(opt->repo);
      
     -  opt->ancestor = "merged common ancestors";
    -+  switch (num_merge_bases) {
    -+  case 0:
    -+          opt->ancestor = "<empty tree>";
    -+          break;
    -+  case 1:
    -+          strbuf_add_unique_abbrev(&merge_base_abbrev,
    -+                                   &merged_common_ancestors->object.oid,
    -+                                   DEFAULT_ABBREV);
    -+          opt->ancestor = merge_base_abbrev.buf;
    -+          break;
    -+  default:
    -+          opt->ancestor = "merged common ancestors";
    -+  }
    ++  opt->ancestor = ancestor_name;
        clean = merge_trees(opt, get_commit_tree(h1), get_commit_tree(h2),
                            get_commit_tree(merged_common_ancestors),
                            &mrtree);
    @@ t/t6047-diff3-conflict-markers.sh (new)
     +
     +          test_must_fail git -c merge.conflictstyle=diff3 merge 
--allow-unrelated-histories -s recursive R^0 &&
     +
    -+          grep "|||||| <empty tree>" content
    ++          grep "|||||| empty tree" content
     +  )
     +'
     +
 5:  19ff6a9503 =  5:  9c0bab2a0b merge-recursive: introduce an enum for 
detect_directory_renames values
 6:  5e44146da1 =  6:  2cd1a60a16 merge-recursive: future-proof 
update_file_flags() against memory leaks
 7:  df210eb029 !  7:  27a339781f merge-recursive: remove another implicit 
dependency on the_repository
    @@ Commit message
     
      ## merge-recursive.c ##
     @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
    -   default:
    -           opt->ancestor = "merged common ancestors";
    -   }
    +           repo_read_index(opt->repo);
    + 
    +   opt->ancestor = ancestor_name;
     -  clean = merge_trees(opt, get_commit_tree(h1), get_commit_tree(h2),
     -                      get_commit_tree(merged_common_ancestors),
     +  clean = merge_trees(opt,
 8:  74dd7b8f59 =  8:  133c470371 Ensure index matches head before invoking 
merge machinery, round N
 9:  f04eba4184 !  9:  f43a229ae6 merge-recursive: exit early if index != head
    @@ merge-recursive.c: static struct commit_list *reverse_commit_list(struct 
commit_
        struct commit_list *iter;
        struct commit *merged_common_ancestors;
     @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
    -   int num_merge_bases;
    +   const char *ancestor_name;
        struct strbuf merge_base_abbrev = STRBUF_INIT;
      
     -  if (!opt->call_depth)
    @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
                        return -1;
                opt->branch1 = saved_b1;
     @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
    -   default:
    -           opt->ancestor = "merged common ancestors";
    -   }
    +           repo_read_index(opt->repo);
    + 
    +   opt->ancestor = ancestor_name;
     -  clean = merge_trees(opt,
     -                      repo_get_commit_tree(opt->repo, h1),
     -                      repo_get_commit_tree(opt->repo, h2),
10:  8688d40739 = 10:  678dfbf507 merge-recursive: remove useless parameter in 
merge_trees()
11:  a92d460707 = 11:  7b28da5637 merge-recursive: don't force external callers 
to do our logging
12:  dec0ea7409 ! 12:  c1ae5cc1c4 cache-tree: share code between functions 
writing an index as a tree
    @@ builtin/checkout.c: static int merge_working_tree(const struct 
checkout_opts *op
                        init_merge_options(&o, the_repository);
                        o.verbosity = 0;
     -                  work = write_tree_from_memory(&o);
    -+                  work = write_inmemory_index_as_tree(the_repository);
    ++                  work = write_in_core_index_as_tree(the_repository);
      
                        ret = reset_tree(new_tree,
                                         opts, 1,
    @@ cache-tree.c: static struct cache_tree *cache_tree_find(struct 
cache_tree *it, c
        return it;
      }
      
    -+int write_index_as_tree_internal(struct object_id *oid, struct 
index_state *index_state, int cache_tree_valid, int flags, const char *prefix)
    ++static int write_index_as_tree_internal(struct object_id *oid,
    ++                                  struct index_state *index_state,
    ++                                  int cache_tree_valid,
    ++                                  int flags,
    ++                                  const char *prefix)
     +{
     +  if (flags & WRITE_TREE_IGNORE_CACHE_TREE) {
     +          cache_tree_free(&index_state->cache_tree);
    @@ cache-tree.c: static struct cache_tree *cache_tree_find(struct 
cache_tree *it, c
     +  return 0;
     +}
     +
    -+struct tree* write_inmemory_index_as_tree(struct repository *repo) {
    ++struct tree* write_in_core_index_as_tree(struct repository *repo) {
     +  struct object_id o;
     +  int was_valid, ret;
     +
    @@ cache-tree.h: void cache_tree_verify(struct repository *, struct 
index_state *);
      #define WRITE_TREE_UNMERGED_INDEX (-2)
      #define WRITE_TREE_PREFIX_ERROR (-3)
      
    -+struct tree* write_inmemory_index_as_tree(struct repository *repo);
    ++struct tree* write_in_core_index_as_tree(struct repository *repo);
      int write_index_as_tree(struct object_id *oid, struct index_state 
*index_state, const char *index_path, int flags, const char *prefix);
      void prime_cache_tree(struct repository *, struct index_state *, struct 
tree *);
      
    @@ merge-recursive.c: static int merge_trees_internal(struct merge_options 
*opt,
      
     -  if (opt->call_depth && !(*result = write_tree_from_memory(opt)))
     +  if (opt->call_depth &&
    -+      !(*result = write_inmemory_index_as_tree(opt->repo)))
    ++      !(*result = write_in_core_index_as_tree(opt->repo)))
                return -1;
      
        return clean;
13:  b51f3d1924 ! 13:  2e9fb223d9 merge-recursive: fix some overly long lines
    @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt
     -          merged_common_ancestors = make_virtual_commit(opt->repo, tree, 
"ancestor");
     +          merged_common_ancestors = make_virtual_commit(opt->repo,
     +                                                        tree, "ancestor");
    -   }
    - 
    -   for (iter = ca; iter; iter = iter->next) {
    +           ancestor_name = "empty tree";
    +   } else if (ca) {
    +           ancestor_name = "merged common ancestors";
     @@ merge-recursive.c: int merge_recursive(struct merge_options *opt,
        return clean;
      }
14:  a069cc4cca ! 14:  230d903012 merge-recursive: use common name for 
ancestors/common/base_list
    @@ merge-recursive.c: static struct commit_list *reverse_commit_list(struct 
commit_
     +  struct commit *merged_merge_bases;
        struct tree *mrtree;
        int clean;
    -   int num_merge_bases;
    +   const char *ancestor_name;
     @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt,
                output_commit_title(opt, h2);
        }
    @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt
                        output_commit_title(opt, iter->item);
        }
      
    --  num_merge_bases = commit_list_count(ca);
     -  merged_common_ancestors = pop_commit(&ca);
     -  if (merged_common_ancestors == NULL) {
    -+  num_merge_bases = commit_list_count(merge_bases);
     +  merged_merge_bases = pop_commit(&merge_bases);
     +  if (merged_merge_bases == NULL) {
                /* if there is no common ancestor, use an empty tree */
    @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt
     -                                                        tree, "ancestor");
     +          merged_merge_bases = make_virtual_commit(opt->repo, tree,
     +                                                   "ancestor");
    +           ancestor_name = "empty tree";
    +-  } else if (ca) {
    ++  } else if (merge_bases) {
    +           ancestor_name = "merged common ancestors";
    +   } else {
    +           strbuf_add_unique_abbrev(&merge_base_abbrev,
    +-                                   &merged_common_ancestors->object.oid,
    ++                                   &merged_merge_bases->object.oid,
    +                                    DEFAULT_ABBREV);
    +           ancestor_name = merge_base_abbrev.buf;
        }
      
     -  for (iter = ca; iter; iter = iter->next) {
    @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt
                        return err(opt, _("merge returned no commit"));
        }
      
    -@@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt,
    -           break;
    -   case 1:
    -           strbuf_add_unique_abbrev(&merge_base_abbrev,
    --                                   &merged_common_ancestors->object.oid,
    -+                                   &merged_merge_bases->object.oid,
    -                                    DEFAULT_ABBREV);
    -           opt->ancestor = merge_base_abbrev.buf;
    -           break;
     @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt,
                                     repo_get_commit_tree(opt->repo, h1),
                                     repo_get_commit_tree(opt->repo, h2),
15:  93a3ce6b88 ! 15:  28c20a2d08 merge-recursive: rename 'mrtree' to 
'result_tree', for clarity
    @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt
     -  struct tree *mrtree;
     +  struct tree *result_tree;
        int clean;
    -   int num_merge_bases;
    +   const char *ancestor_name;
        struct strbuf merge_base_abbrev = STRBUF_INIT;
     @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt,
                                     repo_get_commit_tree(opt->repo, h2),
16:  1b1df10c11 = 16:  50e2e51cf6 merge-recursive: rename merge_options 
argument to opt in header
17:  1526977a85 = 17:  b8cd111bd2 merge-recursive: move some definitions around 
to clean up the header
18:  c90f2f15cd ! 18:  9bb8a7f162 merge-recursive: consolidate unnecessary 
fields in merge_options
    @@ merge-recursive.c: static int add_cacheinfo(struct merge_options *opt,
      {
     -  return opt->merge_detect_rename >= 0 ? opt->merge_detect_rename :
     -          opt->diff_detect_rename >= 0 ? opt->diff_detect_rename : 1;
    -+  return (opt->detect_renames != -1) ? opt->detect_renames : 1;
    ++  return (opt->detect_renames >= 0) ? opt->detect_renames : 1;
      }
      
      static void init_tree_desc_from_tree(struct tree_desc *desc, struct tree 
*tree)
    @@ merge-recursive.c: static struct diff_queue_struct *get_diffpairs(struct 
merge_o
     -  opts.rename_limit = opt->merge_rename_limit >= 0 ? 
opt->merge_rename_limit :
     -                      opt->diff_rename_limit >= 0 ? 
opt->diff_rename_limit :
     -                      1000;
    -+  opts.rename_limit = (opt->rename_limit != -1) ? opt->rename_limit : 
1000;
    ++  opts.rename_limit = (opt->rename_limit >= 0) ? opt->rename_limit : 1000;
        opts.rename_score = opt->rename_score;
        opts.show_rename_progress = opt->show_rename_progress;
        opts.output_format = DIFF_FORMAT_NO_OUTPUT;
19:  6d930dba72 = 19:  2140490796 merge-recursive: comment and reorder the 
merge_options fields
20:  ec3e15f6a8 = 20:  02478e2088 merge-recursive: avoid losing output and 
leaking memory holding that output
21:  7edfac7048 ! 21:  2781ca78d7 merge-recursive: split internal fields into a 
separate struct
    @@ merge-recursive.c: static int merge_trees_internal(struct merge_options 
*opt,
      
     -  if (opt->call_depth &&
     +  if (opt->priv->call_depth &&
    -       !(*result = write_inmemory_index_as_tree(opt->repo)))
    +       !(*result = write_in_core_index_as_tree(opt->repo)))
                return -1;
      
     @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt,
    @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt
     +  if (!opt->priv->call_depth)
                repo_read_index(opt->repo);
      
    -   switch (num_merge_bases) {
    +   opt->ancestor = ancestor_name;
     @@ merge-recursive.c: static int merge_recursive_internal(struct 
merge_options *opt,
                return clean;
        }
    @@ merge-recursive.c: static int merge_start(struct merge_options *opt, 
struct tree
                diff_warn_rename_limit("merge.renamelimit",
     -                                 opt->needed_rename_limit, 0);
     +                                 opt->priv->needed_rename_limit, 0);
    -+  free(opt->priv);
    -+  opt->priv = NULL;
    ++  FREE_AND_NULL(opt->priv);
      }
      
      int merge_trees(struct merge_options *opt,
22:  9a381873c2 = 22:  5a52aa786e merge-recursive: rename MERGE_RECURSIVE_* to 
MERGE_VARIANT_*
23:  c6bc8a196f ! 23:  b7e6c2436a merge-recursive: add sanity checks for 
relevant merge_options
    @@ merge-recursive.c: static int merge_start(struct merge_options *opt, 
struct tree
     +         opt->detect_directory_renames <= MERGE_DIRECTORY_RENAMES_TRUE);
     +  assert(opt->rename_limit >= -1);
     +  assert(opt->rename_score >= 0 && opt->rename_score <= MAX_SCORE);
    ++  assert(opt->show_rename_progress >= 0 && opt->show_rename_progress <= 
1);
     +
     +  assert(opt->xdl_opts >= 0);
     +  assert(opt->recursive_variant >= MERGE_VARIANT_NORMAL &&
     +         opt->recursive_variant <= MERGE_VARIANT_THEIRS);
     +
     +  assert(opt->verbosity >= 0 && opt->verbosity <= 5);
    -+  assert(opt->buffer_output >= 0 && opt->buffer_output <= 2);
    ++  assert(opt->buffer_output <= 2);
     +  assert(opt->obuf.len == 0);
     +
     +  assert(opt->priv == NULL);
    @@ merge-recursive.c: static int merge_start(struct merge_options *opt, 
struct tree
        if (repo_index_has_changes(opt->repo, head, &sb)) {
                err(opt, _("Your local changes to the following files would be 
overwritten by merge:\n  %s"),
                    sb.buf);
    -
    - ## merge-recursive.h ##
    -@@ merge-recursive.h: struct merge_options {
    -   } detect_directory_renames;
    -   int rename_limit;
    -   int rename_score;
    --  int show_rename_progress;
    -+  int show_rename_progress : 1;
    - 
    -   /* xdiff-related options (patience, ignore whitespace, ours/theirs) */
    -   long xdl_opts;
24:  2123e9e4e4 = 24:  6ac9e42a3e merge-recursive: alphabetize include list
-- 
2.23.0.rc2.28.g5f89f15d7b.dirty

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