alloc: add repository argument to alloc_tree_node

This is a small mechanical change; it doesn't change the
implementation to handle repositories other than the_repository yet.
Use a macro to catch callers passing a repository other than
the_repository at compile time.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree.c

@@ -197,7 +197,7 @@ struct tree *lookup_tree(const struct object_id *oid)
 	struct object *obj = lookup_object(oid->hash);
 	if (!obj)
 		return create_object(the_repository, oid->hash,
-				     alloc_tree_node());
+				     alloc_tree_node(the_repository));
 	return object_as_type(obj, OBJ_TREE, 0);
 }