[PATCH] Fix hiding a message while some citations are visible

[Dmitry Kurochkin]

This patch series does some fixes and improvements for hiding and
showing messages in notmuch-show.  However it comes with a
regression: isearch is broken for hidden overlays when
`invisible' property is a list.  I have opened emacs bug #8721
[1] and sent a patch.  It has been committed to emacs trunk
r104356.

I do not know if this should be pushed or wait until emacs with
the fix is released.

Regards,
  Dmitry

[1] http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8721