commit.c: make compare_commits_by_commit_date global

This helper function was introduced as a prio_queue
comparator to help topological sorting. However, other users
of prio_queue who want to replace commit_list_insert_by_date
will want to use it, too. So let's make it public.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -581,7 +581,7 @@ static int compare_commits_by_author_date(const void *a_, const void *b_,
 	return 0;
 }
 
-static int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused)
+int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused)
 {
 	const struct commit *a = a_, *b = b_;
 	/* newer commits with larger date first */

commit.h

@@ -254,4 +254,6 @@ extern void print_commit_list(struct commit_list *list,
  */
 extern void check_commit_signature(const struct commit* commit, struct signature_check *sigc);
 
+int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused);
+
 #endif /* COMMIT_H */