verify-tag was executing an entirely different codepath than
verify-commit, except for the underlying verify_signed_buffer. Move
much of the code from check_commit_signature to a generic
check_signature function and adjust both codepaths to call it.
Update verify-tag to explicitly output the signature text, as we now
call verify_signed_buffer with strbufs to catch the output, which
prevents it from being printed automatically.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>