[MERGE] 1.12-preview formats

[Aaron Bentley]

Ian Clatworthy wrote:
> For the sake of code clarity, I've introduced this new format
> in a new file - workingtree_5.py - and the new classes
> subclass their wt4 counterparts. That's consistent with
> how other workingtrees have been introduced fwiw. In theory,
> it ought to be ever so slightly faster to make wt5 the
> complex class and for wt4 to subclass it - the approach we
> take with branch & repo formats. I personally don't think
> the performance gain is worth the trouble/complexity but
> I'm open to reconsidering this if others think otherwise.

Performance is not the reason we do it.  We do it so that
1. New formats are not dependant on older ones.  This means older
formats can be decomissioned as needed.

2. We care more about newer formats, especially as times goes on and
they become default formats.  This keeps their code consolidated in one
place.

It's also worth bearing in mind the fact that none of our working tree
formats were introduced since we changed our procedures to make older
formats subclass newer ones.

Aaron