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/cache-tree.c b/cache-tree.c
index 6b46711..181d591 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -671,7 +671,8 @@ static void prime_cache_tree_rec(struct cache_tree *it, struct tree *tree)
 			cnt++;
 		else {
 			struct cache_tree_sub *sub;
-			struct tree *subtree = lookup_tree(entry.oid);
+			struct tree *subtree = lookup_tree(the_repository,
+							   entry.oid);
 			if (!subtree->object.parsed)
 				parse_tree(subtree);
 			sub = cache_tree_sub(it, entry.path);