emacs/tree use notmuch-show-single-message

This is more efficient than notmuch-show-only-matching-messages, since
we do not parse the potentially large thread structure to find a
single message.

This is only a partial fix for notmuch-tree view, because displaying
the thread structure in the tree-mode window still crashes on long
threads. It is however enough to make unthreaded view handle long
threads.