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/sequencer.c b/sequencer.c
index 0a291c9..4cf7afb 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -433,7 +433,7 @@ static int read_oneliner(struct strbuf *buf,
 
 static struct tree *empty_tree(void)
 {
-	return lookup_tree(the_hash_algo->empty_tree);
+	return lookup_tree(the_repository, the_repository->hash_algo->empty_tree);
 }
 
 static int error_dirty_index(struct replay_opts *opts)