tag: stop using the_repository

tag: stop using the_repository

gpg_verify_tag() shows the passed in object name on error.  Both callers
provide one.  It falls back to abbreviated hashes for future callers
that pass in a NULL name.  DEFAULT_ABBREV is default_abbrev, which in
turn is a global variable that's populated by git_default_config() and
only available with USE_THE_REPOSITORY_VARIABLE.

Don't let that hypothetical hold us back from getting rid of
the_repository in tag.c.  Fall back to full hashes, which are more
appropriate for error messages anyway.  This allows us to stop setting
USE_THE_REPOSITORY_VARIABLE.

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