bisect: drop unparse_commit() and use clear_commit_marks()

The goal of this patch series is to check if good revisions are
ancestor of the bad revision without forking a process to launch
"git rev-list $good ^$bad".

This new version of this patch series does not use an "unparse_commit"
function anymore, we use "clear_commit_marks" instead.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -771,7 +771,7 @@ static int check_ancestors(const char *prefix)
 	/* Clean up objects used, as they will be reused. */
 	for (i = 0; i < pending_copy.nr; i++) {
 		struct object *o = pending_copy.objects[i].item;
-		unparse_commit((struct commit *)o);
+		clear_commit_marks((struct commit *)o, ALL_REV_FLAGS);
 	}
 
 	return res;