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: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>