string-list: use QSORT_S in string_list_sort()

Pass the comparison function to cmp_items() via the context parameter of
qsort_s() instead of using a global variable.  That allows calling
string_list_sort() from multiple parallel threads.

Our qsort_s() in compat/ is slightly slower than qsort(1) from glibc
2.24 for sorting lots of lines:

Test                         HEAD^             HEAD
---------------------------------------------------------------------
0071.2: sort(1)              0.10(0.22+0.01)   0.09(0.21+0.00) -10.0%
0071.3: string_list_sort()   0.16(0.15+0.01)   0.17(0.15+0.00) +6.3%

GNU sort(1) version 8.26 is significantly faster because it uses
multiple parallel threads; with the unportable option --parallel=1 it
becomes slower:

Test                         HEAD^             HEAD
--------------------------------------------------------------------
0071.2: sort(1)              0.21(0.18+0.01)   0.20(0.18+0.01) -4.8%
0071.3: string_list_sort()   0.16(0.13+0.02)   0.17(0.15+0.01) +6.3%

There is some instability -- the numbers for the sort(1) check shouldn't
be affected by this patch.  Anyway, the performance of our qsort_s()
implementation is apparently good enough, at least for this test.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

string-list.c

@@ -211,21 +211,18 @@ struct string_list_item *string_list_append(struct string_list *list,
 			list->strdup_strings ? xstrdup(string) : (char *)string);
 }
 
-/* Yuck */
-static compare_strings_fn compare_for_qsort;
-
-/* Only call this from inside string_list_sort! */
-static int cmp_items(const void *a, const void *b)
+static int cmp_items(const void *a, const void *b, void *ctx)
 {
+	compare_strings_fn cmp = ctx;
 	const struct string_list_item *one = a;
 	const struct string_list_item *two = b;
-	return compare_for_qsort(one->string, two->string);
+	return cmp(one->string, two->string);
 }
 
 void string_list_sort(struct string_list *list)
 {
-	compare_for_qsort = list->cmp ? list->cmp : strcmp;
-	QSORT(list->items, list->nr, cmp_items);
+	QSORT_S(list->items, list->nr, cmp_items,
+		list->cmp ? list->cmp : strcmp);
 }
 
 struct string_list_item *unsorted_string_list_lookup(struct string_list *list,