When considering a rename for two files that have a suffix and a prefix
that can overlap, a confusing line is shown. As an example, renaming
"a/b/b/c" to "a/b/c" shows "a/b/{ => }/b/c", instead of "a/b/{b => }/c"

Currently, what we do is calculate the common prefix ("a/b/"), and the
common suffix ("/b/c"), but the same "/b/" is actually counted both in
prefix and suffix. Then when calculating the size of the non-common part,
we end-up with a negative value which is reset to 0, thus the "{ => }".

Do not allow the common suffix to overlap the common prefix and stop
when reaching a "/" that would be in both.

Also add some test file to place corner-cases we could met (and this one)
with rename pretty print.

Signed-off-by: Antoine Pelisse <apeli...@gmail.com>
---
 diff.c                   |   11 +++++++++-
 t/t4056-rename-pretty.sh |   54 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+), 1 deletion(-)
 create mode 100755 t/t4056-rename-pretty.sh

diff --git a/diff.c b/diff.c
index 9038f19..e1d82c9 100644
--- a/diff.c
+++ b/diff.c
@@ -1177,7 +1177,16 @@ static char *pprint_rename(const char *a, const char *b)
        old = a + len_a;
        new = b + len_b;
        sfx_length = 0;
-       while (a <= old && b <= new && *old == *new) {
+       /*
+        * Note:
+        * if pfx_length is 0, old/new will never reach a - 1 because it
+        * would mean the whole string is common suffix. But then, the
+        * whole string would also be a common prefix, and we would not
+        * have pfx_length equals 0.
+        */
+       while (a + pfx_length - 1 <= old &&
+              b + pfx_length - 1 <= new &&
+              *old == *new) {
                if (*old == '/')
                        sfx_length = len_a - (old - a);
                old--;
diff --git a/t/t4056-rename-pretty.sh b/t/t4056-rename-pretty.sh
new file mode 100755
index 0000000..806046f
--- /dev/null
+++ b/t/t4056-rename-pretty.sh
@@ -0,0 +1,54 @@
+#!/bin/sh
+
+test_description='Rename pretty print
+
+'
+
+. ./test-lib.sh
+
+test_expect_success nothing_common '
+       mkdir -p a/b/ &&
+       : >a/b/c &&
+       git add a/b/c &&
+       git commit -m. &&
+       mkdir -p c/b/ &&
+       git mv a/b/c c/b/a &&
+       git commit -m. &&
+       git show -M --summary >output &&
+       test_i18ngrep "a/b/c => c/b/a" output
+'
+
+test_expect_success common_prefix '
+       mkdir -p c/d &&
+       git mv c/b/a c/d/e &&
+       git commit -m. &&
+       git show -M --summary >output &&
+       test_i18ngrep "c/{b/a => d/e}" output
+'
+
+test_expect_success common_suffix '
+       mkdir d &&
+       git mv c/d/e d/e &&
+       git commit -m. &&
+       git show -M --summary >output &&
+       test_i18ngrep "{c/d => d}/e" output
+'
+
+test_expect_success common_suffix_prefix '
+       mkdir d/f &&
+       git mv d/e d/f/e &&
+       git commit -m. &&
+       git show -M --summary >output &&
+       test_i18ngrep "d/{ => f}/e" output
+'
+
+test_expect_success common_overlap '
+       mkdir d/f/f &&
+       git mv d/f/e d/f/f/e &&
+       git commit -m. &&
+       git show -M --summary >output &&
+       test_i18ngrep "d/f/{ => f}/e" output
+'
+
+
+test_done
--
1.7.9.5

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