verify-tag: share code with verify-commit

verify-tag: share code with verify-commit

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>