Teach fsck-cache to accept non-commits for reachability analysis.

In particular, teach it about tags. Also, to make reachability actually
work for tags, we need to add the ref to the tagged object.

fsck-cache.c

@@ -295,14 +295,12 @@ int main(int argc, char **argv)
 			continue;
 
 		if (!get_sha1(arg, head_sha1)) {
-			struct commit *commit = lookup_commit(head_sha1);
-			struct object *obj;
+			struct object *obj = lookup_object(head_sha1);
 
-			/* Error is printed by lookup_commit(). */
-			if (!commit)
+			/* Error is printed by lookup_object(). */
+			if (!obj)
 				continue;
 
-			obj = &commit->object;
 			obj->used = 1;
 			mark_reachable(obj, REACHABLE);
 			heads++;

tag.c

@@ -49,6 +49,8 @@ int parse_tag(struct tag *item)
 		goto err;
 
 	item->tagged = parse_object(object);
+	if (item->tagged)
+		add_ref(&item->object, item->tagged);
 
 	type_line = data + 48;
 	if (memcmp("\ntype ", type_line-1, 6))