send-email: support separate Reply-To address

send-email: support separate Reply-To address

In some projects contributions from groups are only accepted from a
common group email address. But every individual may want to receive
replies to her own personal address. That's what we have 'Reply-To'
headers for in SMTP. So introduce an optional '--reply-to' command
line option.

This patch re-uses the $reply_to variable. This could break
out-of-tree patches!

Signed-off-by: Christian Ludwig <chrissicool@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>