[MERGE] Make it easier to introduce new WorkingTree formats

[John Arbash Meinel]

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Ian Clatworthy wrote:
| 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.
|

BB:approve

This just reminds me that I *really* wish the "initialize" functions
would return locked objects. As it stands, we usually lock them right
after we get them back, and locking isn't really as cheap as we would
like it to be.

John
=:->

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