commit: use mergesort() in commit_list_sort_by_date()

Replace the insertion sort in commit_list_sort_by_date() with a
call to the generic mergesort function.  This sets the stage for
using commit_list_sort_by_date() for larger lists, as shown in
the next patch.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -7,6 +7,7 @@
 #include "revision.h"
 #include "notes.h"
 #include "gpg-interface.h"
+#include "mergesort.h"
 
 int save_commit_buffer = 1;
 
@@ -390,15 +391,31 @@ struct commit_list * commit_list_insert_by_date(struct commit *item, struct comm
 	return commit_list_insert(item, pp);
 }
 
+static int commit_list_compare_by_date(const void *a, const void *b)
+{
+	unsigned long a_date = ((const struct commit_list *)a)->item->date;
+	unsigned long b_date = ((const struct commit_list *)b)->item->date;
+	if (a_date < b_date)
+		return 1;
+	if (a_date > b_date)
+		return -1;
+	return 0;
+}
+
+static void *commit_list_get_next(const void *a)
+{
+	return ((const struct commit_list *)a)->next;
+}
+
+static void commit_list_set_next(void *a, void *next)
+{
+	((struct commit_list *)a)->next = next;
+}
 
 void commit_list_sort_by_date(struct commit_list **list)
 {
-	struct commit_list *ret = NULL;
-	while (*list) {
-		commit_list_insert_by_date((*list)->item, &ret);
-		*list = (*list)->next;
-	}
-	*list = ret;
+	*list = mergesort(*list, commit_list_get_next, commit_list_set_next,
+			  commit_list_compare_by_date);
 }
 
 struct commit *pop_most_recent_commit(struct commit_list **list,