WorkingTree._control_files should be public

[John Arbash Meinel]

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https://launchpad.net/products/bzr/+bug/50919

Brings up the fact that _control_files is a private member for
WorkingTree, though it is a public member for both Repository and Branch.

And more importantly, TreeTransform needs access to control_files,
because it creates a 'limbo' directory.

Alternatively, we could make a function that creates a limbo directory,
if we actually want to make all the control_files members private.

John
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