emacs: hello: stop update from forcing the buffer to be displayed

Make the notmuch-hello refresh function (notmuch-hello-update) not
force the buffer to be displayed. All the callers call it when the
buffer is already displayed so it will only affect non-interactive
callers. Since it is just a trivial wrapper of notmuch-hello anyone
who wants to force the buffer to be displayed should just call
notmuch-hello.