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/commit.c b/commit.c
index 0d55600..2fa4220 100644
--- a/commit.c
+++ b/commit.c
@@ -383,7 +383,7 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 	if (get_oid_hex(bufptr + 5, &parent) < 0)
 		return error("bad tree pointer in commit %s",
 			     oid_to_hex(&item->object.oid));
-	item->maybe_tree = lookup_tree(&parent);
+	item->maybe_tree = lookup_tree(the_repository, &parent);
 	bufptr += tree_entry_len + 1; /* "tree " + "hex sha1" + "\n" */
 	pptr = &item->parents;