[MERGE] Make it easier to introduce new WorkingTree formats

[Ian Clatworthy]

The attached patch makes it easier to introduce new
WorkingTreeFormats that subclass WorkingTreeFormat4.
Note that WorkingTreeFormat3 already uses the same
_tree_class "trick" but WTF4 doesn't until now.

Introducing a new WorkingTreeFormat is a requirement
to support filtered views. Robert is also requesting
(or insisting on) this for the content-filtering
feature as well. It might help make life easier for
plugins authors trying cool stuff. In short, I think
it is a good thing to make our code easier to extend
and this helps.

Ian C.
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