format-patch: do not abuse 3-dash marker line.

Before GIT version at the end of output we used a 3-dash marker;
but 3-dash marker is special and should not be overused.
Instead, use "-- " which is a standard practice in e-mails to
signal the beginning of trailing garbage.

Signed-off-by: Junio C Hamano <junkio@cox.net>