send-email: more meaningful Message-ID

send-email: more meaningful Message-ID

Using a YYYYmmddHHMMSS date representation is more meaningful to
humans, especially when used for lookups on NNTP servers or linking
to archive sites via Message-ID (e.g. mid.gmane.org or
mid.mail-archive.com).  This timestamp format more easily gives a
reader of the URL itself a rough date of a linked message compared
to having them calculate the seconds since the Unix epoch.

Furthermore, having the MUA name in the Message-ID seems to be a
rare oddity I haven't noticed outside of git-send-email.  We
already have an optional X-Mailer header field to advertise for
us, so extending the Message-ID by 15 characters can make for
unpleasant Message-ID-based URLs to archive sites.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>