Change my mind about how to extend backwards.

Change my mind about how to extend backwards.

I think pressing [0] at the top of the file is better than double-[B].
Firstly, it's fewer keystrokes to do on purpose; secondly, it's not a
key you would have been repeating already. Now I can hold down [B]
safely to page up through the existing text, _and_ if I want to keep
extending backwards I can do it more easily than before.