Do not show diff help message when in do not disturb mode and in different frame or tab

[stevemolitor]

When monet-do-not-disturb it t, we don't pop open the diff buffer when the user is in a different tab or frame. However, we show this message in the minibuffer:

Type in the diff buffer to accept changes, or q to reject.

That's not the end of the world - it's still much less invasive than before. But we should either not show that message, or display a different message, when in a different tab or frame, and do not disturb is on.