First, I'm aware that such munging is A Bad Thing

  http://www.unicom.com/pw/reply-to-harmful.html

but a lot of lists do it anyway (mostly to work around widely used
mailers with lame defaults).  After munging, we get headers looking like
this

  From: Some User <some.user@example.com>
  To: Sample users list <sample-users@sample.org>
  Reply-To: Sample users list <sample-users@sample.org>
  
Notmuch reply produces

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

Handling this is a bit messy, I think we want the current behavior
unless To matches Reply-To, in which case we use From and Reply-To.  If
this is indeed the least bad behavior, I will make a patch for it.

Jed