walker: avoid quadratic list insertion in mark_complete

Similar to 1644524 (fetch-pack: avoid quadratic list insertion in
mark_complete), sort only after all refs are collected instead of while
inserting.  The result is the same, but it's more efficient that way.
The difference will only be measurable in repositories with a large
number of refs.

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

walker.c

@@ -204,7 +204,7 @@ static int mark_complete(const char *path, const unsigned char *sha1, int flag,
 	struct commit *commit = lookup_commit_reference_gently(sha1, 1);
 	if (commit) {
 		commit->object.flags |= COMPLETE;
-		commit_list_insert_by_date(commit, &complete);
+		commit_list_insert(commit, &complete);
 	}
 	return 0;
 }
@@ -269,8 +269,10 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 		}
 	}
 
-	if (!walker->get_recover)
+	if (!walker->get_recover) {
 		for_each_ref(mark_complete, NULL);
+		commit_list_sort_by_date(&complete);
+	}
 
 	for (i = 0; i < targets; i++) {
 		if (interpret_target(walker, target[i], &sha1[20 * i])) {