emacs: Fix navigation of multi-line search result formats

emacs: Fix navigation of multi-line search result formats

At this point, the only remaining functions that don't support
multi-line search result formats are the thread navigation functions.
This patch fixes that by rewriting them in terms of
notmuch-search-result-{beginning,end}.

This changes the behavior of notmuch-search-previous-thread slightly
so that if point isn't at the beginning of a result, it first moves
point to the beginning of the result.