Git's rename detection requires a stable sort

Makefile

@@ -983,6 +983,7 @@ LIB_OBJS += shallow.o
 LIB_OBJS += sideband.o
 LIB_OBJS += sigchain.o
 LIB_OBJS += split-index.o
+LIB_OBJS += stable-qsort.o
 LIB_OBJS += strbuf.o
 LIB_OBJS += streaming.o
 LIB_OBJS += string-list.o
@@ -1714,7 +1715,6 @@ ifdef NO_GETPAGESIZE
 endif
 ifdef INTERNAL_QSORT
 	COMPAT_CFLAGS += -DINTERNAL_QSORT
-	COMPAT_OBJS += compat/qsort.o
 endif
 ifdef HAVE_ISO_QSORT_S
 	COMPAT_CFLAGS += -DHAVE_ISO_QSORT_S

compat/mingw.c

@@ -1229,11 +1229,6 @@ static int wenvcmp(const void *a, const void *b)
 	return _wcsnicmp(p, q, p_len);
 }
 
-/* We need a stable sort to convert the environment between UTF-16 <-> UTF-8 */
-#ifndef INTERNAL_QSORT
-#include "qsort.c"
-#endif
-
 /*
  * Build an environment block combining the inherited environment
  * merged with the given list of settings.
@@ -1272,8 +1267,8 @@ static wchar_t *make_environment_block(char **deltaenv)
 
 	/*
 	 * If there is a deltaenv, let's accumulate all keys into `array`,
-	 * sort them using the stable git_qsort() and then copy, skipping
-	 * duplicate keys
+	 * sort them using the stable git_stable_qsort() and then copy,
+	 * skipping duplicate keys
 	 */
 	for (p = wenv; p && *p; ) {
 		ALLOC_GROW(array, nr + 1, alloc);
@@ -1296,7 +1291,7 @@ static wchar_t *make_environment_block(char **deltaenv)
 		p += wlen + 1;
 	}
 
-	git_qsort(array, nr, sizeof(*array), wenvcmp);
+	git_stable_qsort(array, nr, sizeof(*array), wenvcmp);
 	ALLOC_ARRAY(result, size + delta_size);
 
 	for (p = result, i = 0; i < nr; i++) {

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)

git-compat-util.h

@@ -1094,10 +1094,10 @@ static inline int strtol_i(char const *s, int base, int *result)
 	return 0;
 }
 
+void git_stable_qsort(void *base, size_t nmemb, size_t size,
+		      int(*compar)(const void *, const void *));
 #ifdef INTERNAL_QSORT
-void git_qsort(void *base, size_t nmemb, size_t size,
-	       int(*compar)(const void *, const void *));
-#define qsort git_qsort
+#define qsort git_stable_qsort
 #endif
 
 #define QSORT(base, n, compar) sane_qsort((base), (n), sizeof(*(base)), compar)
@@ -1108,6 +1108,9 @@ static inline void sane_qsort(void *base, size_t nmemb, size_t size,
 		qsort(base, nmemb, size, compar);
 }
 
+#define STABLE_QSORT(base, n, compar) \
+	git_stable_qsort((base), (n), sizeof(*(base)), compar)
+
 #ifndef HAVE_ISO_QSORT_S
 int git_qsort_s(void *base, size_t nmemb, size_t size,
 		int (*compar)(const void *, const void *, void *), void *ctx);

compat/qsort.c → stable-qsort.c

@@ -1,4 +1,4 @@
-#include "../git-compat-util.h"
+#include "git-compat-util.h"
 
 /*
  * A merge sort implementation, simplified from the qsort implementation
@@ -44,8 +44,8 @@ static void msort_with_tmp(void *b, size_t n, size_t s,
 	memcpy(b, t, (n - n2) * s);
 }
 
-void git_qsort(void *b, size_t n, size_t s,
-	       int (*cmp)(const void *, const void *))
+void git_stable_qsort(void *b, size_t n, size_t s,
+		      int (*cmp)(const void *, const void *))
 {
 	const size_t size = st_mult(n, s);
 	char buf[1024];