Allow for having for_each_ref() list extra refs

Allow for having for_each_ref() list extra refs

These refs can be anything, but they are most likely useful as
pointing to objects that you know are in the object database but don't
have any regular refs for. For example, when cloning with --reference,
the refs in this repository should be listed as objects that we have,
even though we don't have refs in our newly-created repository for
them yet.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>