notmuch.el: Don't advance line in search buffer before showing thread.

Previously, when selecting a thread to view from the search buffer, we
would advance the point by one line before showing the thread, (so
that it would be ready to show the next thread once the user was done
with the current thread). This was annoying when the user temporarily
exited the thread view, (because the "wrong" thread was then selected
in the search view).

We get a more consistent experience by waiting to advance until the
user has finished viewing one thread and is ready to view the next.