emacs: Open only matched (and unread) messages when displaying a thread.

emacs: Open only matched (and unread) messages when displaying a thread.

This is the long-awaited feature that when viewing a thread resulting
from a search, only the messages that actually match the search will
be opened initially (in addition to unread messages).

So now, it's finally useful to tag a single message in a giant thread,
and then do a search later and easily find just the single tagged
message.