Add message_id and ref_message_id fields to struct rev_info, used in show_log
with CMIT_FMT_EMAIL to set Message-Id and In-Reply-To/References respectively.
Use these in git-format-patch to make the second and subsequent patch mails
replies to the first patch mail.
Signed-off-by: NJosh Triplett <josh@freedesktop.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>