emacs: interesting-buffer bugfixes/updates

The command notmuch-interesting-buffer has got out of date -- it
doesn't mention notmuch-tree, and it still refers to message-mode not
notmuch-message-mode. Update both of these.

This fixes the bug that notmuch-cycle-notmuch-buffers does not include
notmuch-tree or notmuch-message-mode buffers in its cycling.