send-email: try to order messages in email clients more correctly

send-email: try to order messages in email clients more correctly

If --no-chain-reply-to is set, patches may not always be ordered
correctly in email clients.  This patch makes sure each email
sent from a different second.

I chose to start with a time (slightly) in the past because
those are probably more likely in real-world usage and spam
filters might be more tolerant of them.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>