pack-objects: do not get distracted by broken symrefs

It is quite possible for, say, a remote HEAD to become broken, e.g.
when the default branch was renamed.

We should still be able to pack our objects when such a thing happens;
simply ignore broken symrefs (because they cannot matter for the packing
process anyway).

This fixes https://github.com/git-for-windows/git/issues/423

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reachable.c

@@ -24,9 +24,15 @@ static void update_progress(struct connectivity_progress *cp)
 
 static int add_one_ref(const char *path, const unsigned char *sha1, int flag, void *cb_data)
 {
-	struct object *object = parse_object_or_die(sha1, path);
 	struct rev_info *revs = (struct rev_info *)cb_data;
+	struct object *object;
 
+	if ((flag & REF_ISSYMREF) && (flag & REF_ISBROKEN)) {
+		warning("symbolic ref is dangling: %s", path);
+		return 0;
+	}
+
+	object = parse_object_or_die(sha1, path);
 	add_pending_object(revs, object, "");
 
 	return 0;

t/t6500-gc.sh

@@ -30,7 +30,7 @@ test_expect_success 'gc -h with invalid configuration' '
 	test_i18ngrep "[Uu]sage" broken/usage
 '
 
-test_expect_failure 'gc is not aborted due to a stale symref' '
+test_expect_success 'gc is not aborted due to a stale symref' '
 	git init remote &&
 	(
 		cd remote &&