Here's something that I was playing with this morning. It's not
finished, but has some nasty corners, so I'd like to solicit opinion on
whether it's worth finishing.

When faced with a long thread where most of the messages are closed, it
might be convenient to hide large stretches of closed headers. This code
attempts to do that.

You can run `dme/notmuch-show-compress-headers' manually in a
`notmuch-show-mode' buffer or (for the terminally brave) attach the same
function to `notmuch-show-hook'.

There are still problems with the nested invisibility specs and the
current `notmuch-show-advance-and-archive' and friends get confused, but
it should be enough for you to get the idea.

After using it for a bit I came to the conclusion that it's not worth
pursuing, but figured that I'd post it to get feedback from others.