format-patch: show base info before email signature

format-patch: show base info before email signature

Any text below the "-- " for the email signature gets treated as part of
the signature, and many mail clients will trim it from the quoted text
for a reply.  Move it above the signature, so people can reply to it
more easily.

Similarly, when producing the patch as a MIME attachment, the
original code placed the base info after the attached part, which
would be discarded.  Move the base info to the end of the part,
still inside the part boundary.

Add tests for the exact format of the email signature, and add tests
to ensure that the base info appears before the email signature when
producing a plain-text output, and that it appears before the part
boundary when producing a MIME attachment.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>