emacs/tree: add notmuch-tree-matching-message

This functions removes some duplicate logic between
notmuch-tree-{next,prev}-matching-message

We do this because we will be adding some additional logic similar to
the notmuch-show-next-open-message function, and it will help if this
logic is all in one place.

Signed-off-by: William Casarin <jb55@jb55.com>