WorkingTree._control_files should be public

[Aaron Bentley]

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Tree Transform needs to create a directory in the Working Tree control
directory as a temporary storage area.  It shouldn't use a temp directory

1. Because it should FAIL if there are leftovers from a failed transform
operation
2. Because a temp directory might be on another filesystem

Future enhancements to conflict handling suggest we'll want
1. A file that lists all conflicts encountered
2. Possibly a directory to hold the merge input files

It's not clear to me that WorkingTree should support
tree.make_limbo_directory() and tree.make_conflict_directory().  If it
should, then we don't need it as a public API.  But it's a useful API to
have, so I think we should make it public.

Aaron
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