[MERGE] versionedfile.py code cleanups

[Ian Clatworthy]

Martin Pool wrote:
> Martin Pool has voted approve.
> Status is now: Approved

Thanks. I'll merge this with a tweak though - _apply_delta() will be
moved into knit.py as discussed with Aaron and Robert on IRC. I'll make
it a static method of KnitVersionedFile (as that's the only place it's
used).

If anyone disagrees, speak up soon. :-)

Ian C.