In an effort to support having an OpenPGP signing key that doesn't
exactly match the name+email in .gitconfig, separately parameterize
the signingkey option allowing it to be explicitly overridden with a
key ID or other similar identifier (such as a matching E-mail
address).
Change-Id: Id577c4479abd2f443f73c630a12e94d3ebe63660
a2a3faf8a7