verify-commit: add option to print raw gpg status information

verify-commit: add option to print raw gpg status information

verify-commit by default displays human-readable output on standard
error.  However, it can also be useful to get access to the raw gpg
status information, which is machine-readable, allowing automated
implementation of signing policy.  Add a --raw option to make
verify-commit produce the gpg status information on standard error
instead of the human-readable format.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>