format-patch: Add a signature option (--signature)

format-patch: Add a signature option (--signature)

By default, git uses the version string as the signature for all
patches output by format-patch. Many employers (mine included)
require the use of a signature on all outgoing mails. In a
format-patch | send-email workflow there isn't an easy way to modify
the signature without breaking the pipe and manually replacing the
version string with the signature required. Instead of doing all that
work, add an option (--signature) and a config variable
(format.signature) to replace the default git version signature when
formatting patches.

This does modify the original behavior of format-patch a bit. First
off the version string is now placed in the cover letter by default.
Secondly, once the configuration variable format.signature is added
to the .config file there is no way to revert back to the default
git version signature. Instead, specifying the --no-signature option
will remove the signature from the patches entirely.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>