commit: add repository argument to lookup_commit_reference

Add a repository argument to allow callers of lookup_commit_reference
to be more specific about which repository to handle. 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: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-tree.c

@@ -11,7 +11,7 @@ static struct rev_info log_tree_opt;
 
 static int diff_tree_commit_oid(const struct object_id *oid)
 {
-	struct commit *commit = lookup_commit_reference(oid);
+	struct commit *commit = lookup_commit_reference(the_repository, oid);
 	if (!commit)
 		return -1;
 	return log_tree_commit(&log_tree_opt, commit);