Merge branch 'ra/send-email-in-reply-to-from-command-line-wins' into master

"git send-email --in-reply-to=<msg>" did not use the In-Reply-To:
header with the value given from the command line, and let it be
overridden by the value on In-Reply-To: header in the messages
being sent out (if exists).

* ra/send-email-in-reply-to-from-command-line-wins:
  send-email: restore --in-reply-to superseding behavior