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>

git-send-email.perl

@@ -19,6 +19,7 @@
 use 5.008;
 use strict;
 use warnings;
+use POSIX qw/strftime/;
 use Term::ReadLine;
 use Getopt::Long;
 use Text::ParseWords;
@@ -910,7 +911,7 @@ my ($message_id_stamp, $message_id_serial);
 sub make_message_id {
 	my $uniq;
 	if (!defined $message_id_stamp) {
-		$message_id_stamp = sprintf("%s-%s", time, $$);
+		$message_id_stamp = strftime("%Y%m%d%H%M%S.$$", gmtime(time));
 		$message_id_serial = 0;
 	}
 	$message_id_serial++;
@@ -925,7 +926,7 @@ sub make_message_id {
 		require Sys::Hostname;
 		$du_part = 'user@' . Sys::Hostname::hostname();
 	}
-	my $message_id_template = "<%s-git-send-email-%s>";
+	my $message_id_template = "<%s-%s>";
 	$message_id = sprintf($message_id_template, $uniq, $du_part);
 	#print "new message id = $message_id\n"; # Was useful for debugging
 }