notmuch-reply.c: Handle munged `Reply-To' headers.

Some mailing lists engage in the evil practice of changing the Reply-To
header so that replies from all mailers go to the list by default, at
the expense of not responding to the person who actually sent the
message.  When this is detected, we reply to `From' and remove the
duplicate response to the mailing list.  Consider a reply to the
following message.

  From: Some User <some.user@example.com>
  To: Sample users list <sample-users@sample.org>
  Reply-To: Sample users list <sample-users@sample.org>
Prior to this patch, `notmuch reply' produces

  To: Sample users list <sample-users@sample.org>,
      Sample users list <sample-users@sample.org>

and after the patch,

  To: Some User <some.user@example.com>,
      Sample users list <sample-users@sample.org>

Signed-off-by: Jed Brown <jed@59A2.org>