verify-tag exits successfully if the signature is good but the key is
untrusted. verify-commit exits unsuccessfully. This divergence in
behavior is unexpected and unwanted. Since verify-tag existed earlier,
add a failing test to have verify-commit share verify-tag's behavior.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>