t/t7004-tag: test handling of rfc1991 signatures

t/t7004-tag: test handling of rfc1991 signatures

Currently, git expects "-----BEGIN PGP SIGNATURE-----" at the beginning of a
signature. But gpg uses "MESSAGE" instead of "SIGNATURE" when used with
the "rfc1991" option. This leads to git's failing to verify it's own
signed tags, among other problems.

Add tests for all code paths (tag -v, tag -l -n largenumber, tag -f
without -m) where signature detection matters.

Reported-by: Stephan Hugel <urschrei@gmail.com>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>