diffcore_rename(): use a stable sort

During Git's rename detection, the file names are sorted. At the moment,
this job is performed by `qsort()`. As that function is not guaranteed
to implement a stable sort algorithm, this can lead to inconsistent
and/or surprising behavior: a rename might be detected differently
depending on the platform where Git was run.

The `qsort()` in MS Visual C's runtime does _not_ implement a stable
sort algorithm, and it even leads to an inconsistency leading to a test
failure in t3030.35 "merge-recursive remembers the names of all base
trees": a different code path than on Linux is taken in the rename
detection of an ambiguous rename between either `e` to `a` or
`a~Temporary merge branch 2_0` to `a` during a recursive merge,
unexpectedly resulting in a clean merge.

Let's use the stable sort provided by `git_stable_qsort()` to avoid this
inconsistency.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-rename.c

@@ -585,7 +585,7 @@ void diffcore_rename(struct diff_options *options)
 	stop_progress(&progress);
 
 	/* cost matrix sorted by most to least similar pair */
-	QSORT(mx, dst_cnt * NUM_CANDIDATE_PER_DST, score_compare);
+	STABLE_QSORT(mx, dst_cnt * NUM_CANDIDATE_PER_DST, score_compare);
 
 	rename_count += find_renames(mx, dst_cnt, minimum_score, 0);
 	if (detect_rename == DIFF_DETECT_COPY)