`git merge-recursive` does a three-way merge between user-specified trees
base, head, and remote.  Since the user is allowed to specify head, we can
not necesarily assume that head == HEAD.

Modify index_has_changes() to take an extra argument specifying the tree
to compare against.  If NULL, it will compare to HEAD.  We then use this
from merge-recursive to make sure we compare to the user-specified head.

Signed-off-by: Elijah Newren <new...@gmail.com>
---
 builtin/am.c                             |  7 ++++---
 cache.h                                  | 11 +++++++----
 merge-recursive.c                        |  2 +-
 read-cache.c                             | 12 ++++++++----
 t/t6044-merge-unrelated-index-changes.sh |  2 +-
 5 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/builtin/am.c b/builtin/am.c
index 24ad3e435..cb8e2b960 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -1763,7 +1763,7 @@ static void am_run(struct am_state *state, int resume)
 
        refresh_and_write_cache();
 
-       if (index_has_changes(&the_index, &sb)) {
+       if (index_has_changes(&the_index, NULL, &sb)) {
                write_state_bool(state, "dirtyindex", 1);
                die(_("Dirty index: cannot apply patches (dirty: %s)"), sb.buf);
        }
@@ -1820,7 +1820,8 @@ static void am_run(struct am_state *state, int resume)
                         * Applying the patch to an earlier tree and merging
                         * the result may have produced the same tree as ours.
                         */
-                       if (!apply_status && !index_has_changes(&the_index, 
NULL)) {
+                       if (!apply_status &&
+                           !index_has_changes(&the_index, NULL, NULL)) {
                                say(state, stdout, _("No changes -- Patch 
already applied."));
                                goto next;
                        }
@@ -1874,7 +1875,7 @@ static void am_resolve(struct am_state *state)
 
        say(state, stdout, _("Applying: %.*s"), linelen(state->msg), 
state->msg);
 
-       if (!index_has_changes(&the_index, NULL)) {
+       if (!index_has_changes(&the_index, NULL, NULL)) {
                printf_ln(_("No changes - did you forget to use 'git add'?\n"
                        "If there is nothing left to stage, chances are that 
something else\n"
                        "already introduced the same changes; you might want to 
skip this patch."));
diff --git a/cache.h b/cache.h
index 125f2767a..b5a75806e 100644
--- a/cache.h
+++ b/cache.h
@@ -218,6 +218,7 @@ struct cache_entry {
 /* Forward structure decls */
 struct pathspec;
 struct child_process;
+struct tree;
 
 /*
  * Copy the sha1 and stat state of a cache entry from one to
@@ -635,12 +636,14 @@ extern void move_index_extensions(struct index_state 
*dst, struct index_state *s
 extern int unmerged_index(const struct index_state *);
 
 /**
- * Returns 1 if istate differs from HEAD, 0 otherwise.  When on an unborn
- * branch, returns 1 if there are entries in istate, 0 otherwise.  If an
- * strbuf is provided, the space-separated list of files that differ will
- * be appended to it.
+ * Returns 1 if istate differs from tree, 0 otherwise.  If tree is NULL,
+ * compares istate to HEAD.  If tree is NULL and on an unborn branch,
+ * returns 1 if there are entries in istate, 0 otherwise.  If an strbuf is
+ * provided, the space-separated list of files that differ will be appended
+ * to it.
  */
 extern int index_has_changes(const struct index_state *istate,
+                            struct tree *tree,
                             struct strbuf *sb);
 
 extern int verify_path(const char *path, unsigned mode);
diff --git a/merge-recursive.c b/merge-recursive.c
index 9c0699be5..855ca39ca 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -3266,7 +3266,7 @@ int merge_trees(struct merge_options *o,
        if (oid_eq(&common->object.oid, &merge->object.oid)) {
                struct strbuf sb = STRBUF_INIT;
 
-               if (!o->call_depth && index_has_changes(&the_index, &sb)) {
+               if (!o->call_depth && index_has_changes(&the_index, head, &sb)) 
{
                        err(o, _("Your local changes to the following files 
would be overwritten by merge:\n  %s"),
                            sb.buf);
                        return -1;
diff --git a/read-cache.c b/read-cache.c
index ad3102083..acc8c390a 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1986,22 +1986,26 @@ int unmerged_index(const struct index_state *istate)
        return 0;
 }
 
-int index_has_changes(const struct index_state *istate, struct strbuf *sb)
+int index_has_changes(const struct index_state *istate,
+                     struct tree *tree,
+                     struct strbuf *sb)
 {
-       struct object_id head;
+       struct object_id cmp;
        int i;
 
        if (istate != &the_index) {
                BUG("index_has_changes cannot yet accept istate != &the_index; 
do_diff_cache needs updating first.");
        }
-       if (!get_oid_tree("HEAD", &head)) {
+       if (tree)
+               cmp = tree->object.oid;
+       if (tree || !get_oid_tree("HEAD", &cmp)) {
                struct diff_options opt;
 
                diff_setup(&opt);
                opt.flags.exit_with_status = 1;
                if (!sb)
                        opt.flags.quick = 1;
-               do_diff_cache(&head, &opt);
+               do_diff_cache(&cmp, &opt);
                diffcore_std(&opt);
                for (i = 0; sb && i < diff_queued_diff.nr; i++) {
                        if (i)
diff --git a/t/t6044-merge-unrelated-index-changes.sh 
b/t/t6044-merge-unrelated-index-changes.sh
index 3876cfa4f..1d43712c5 100755
--- a/t/t6044-merge-unrelated-index-changes.sh
+++ b/t/t6044-merge-unrelated-index-changes.sh
@@ -126,7 +126,7 @@ test_expect_success 'recursive, when merge branch matches 
merge base' '
        test_path_is_missing .git/MERGE_HEAD
 '
 
-test_expect_failure 'merge-recursive, when index==head but head!=HEAD' '
+test_expect_success 'merge-recursive, when index==head but head!=HEAD' '
        git reset --hard &&
        git checkout C^0 &&
 
-- 
2.18.0.137.g2a11d05a6.dirty

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