tree: add repository argument to lookup_tree

Add a repository argument to allow the callers of lookup_tree
to be more specific about which repository to act on. This is a small
mechanical change; it doesn't change the implementation to handle
repositories other than the_repository yet.

As with the previous commits, use a macro to catch callers passing a
repository other than the_repository at compile time.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/reachable.c b/reachable.c
index cc25f01..6e9b810 100644
--- a/reachable.c
+++ b/reachable.c
@@ -88,7 +88,7 @@ static void add_recent_object(const struct object_id *oid,
 		obj = parse_object_or_die(oid, NULL);
 		break;
 	case OBJ_TREE:
-		obj = (struct object *)lookup_tree(oid);
+		obj = (struct object *)lookup_tree(the_repository, oid);
 		break;
 	case OBJ_BLOB:
 		obj = (struct object *)lookup_blob(the_repository, oid);