[MERGE] Restore "bzr revert" on win32

[John Arbash Meinel]

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Lukas's recent change to introduce "_get_one_revision_tree()" ends up
breaking a few cases on win32. The specific issue, is that it can return
"tree.basis_tree()" which is actually a good thing. However, the merge
code seems to be calling "base_tree.lock_read()" before it calls
"target_tree.lock_write()".

I realize a more complete fix needs to happen in the merge code, but
this fix is good anyway. It means that looking up the revision trees
will be cached, etc.

I would add a test case, but I'm pretty sure they are already failing.

Unless we can think of a good way to add a test case that will fail
reasonably under Linux so that this gets caught by the PQM.

(Note that I still have my "make locks exclusive on Linux branch" which
would do just that.)

John
=:->
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