commit: allow suppression of implicit identity advice

commit: allow suppression of implicit identity advice

We now nag the user with a giant warning when their identity
was pulled from the username, hostname, and gecos
information, in case it is not correct. Most users will
suppress this by simply setting up their information
correctly.

However, there may be some users who consciously want to use
that information, because having the value change from host
to host contains useful information. These users can now set
advice.implicitidentity to false to suppress the message.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>