We have always been creating rfc1991 signatures for users with "rfc1991"
in their gpg config but failed to recognize them (tag -l -n largenumber)
and verify them (tag -v, verify-tag).
Make good use of the refactored signature detection and let us recognize
and verify those signatures also.
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>