emacs: Make moving to the previous message move to the previous boundary

Previously, notmuch-show-previous-message would move to the beginning
of the message before the message containing point.  This patch makes
it instead move to the previous message *boundary*.  That is, if point
isn't already at the beginning of the message, it moves to the
beginning of the current message.  This is consistent with
notmuch-show-next-message, which can be thought of as moving to the
next message boundary.  Several people have expressed a preference for
this.