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>

tree.h

@@ -12,7 +12,8 @@ struct tree {
 	unsigned long size;
 };
 
-struct tree *lookup_tree(const struct object_id *oid);
+#define lookup_tree(r, oid) lookup_tree_##r(oid)
+struct tree *lookup_tree_the_repository(const struct object_id *oid);
 
 int parse_tree_buffer(struct tree *item, void *buffer, unsigned long size);