emacs: Take advantage of position-remembering when returning to notmuch-hello

emacs: Take advantage of position-remembering when returning to notmuch-hello

When we go into a search, and then later quit and return to the
notmuch-hello buffer, we want the point to remain in the same position
it was in when we left. So we have to call the position-remembering
notmuch-hello-update rather than notmuch-hello from the continuation.