tag: support arbitrary repositories in parse_tag()

tag: support arbitrary repositories in parse_tag()

Allow callers of parse_tag() pass in the repository to use.  Let most of
them pass in the_repository to get the same result as before.  One of
them has stopped using the_repository in ef9b0370da (sha1-name.c: store
and use repo in struct disambiguate_state, 2019-04-16); let it pass in
its stored repository.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>