commit: add repository argument to lookup_commit_reference_gently

commit: add repository argument to lookup_commit_reference_gently

Add a repository argument to allow callers of
lookup_commit_reference_gently 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>