send-email: make message-id generation a bit more robust

send-email: make message-id generation a bit more robust

Earlier code took Unix time and appended a few random digits.
If you are firing off many messages within a second, you could
issue the same id to different messages, which is a no-no.  If
you send out 31 messages within a single second, with random
integer taken out of rand(4200), you have about 10% chance of
producing the same message ID.

This fixes the problem by uses a prefix string which is
constant-per-invocation (time and pid), with a serial number for
each message generated by the process appended at the end.

Signed-off-by: Junio C Hamano <gitster@pobox.com>