[MERGE] Convert workingtree.py's use of lock_write to lock_tree_write

[Robert Collins]

All WorkingTree methods which write to the tree, but not to the branch
have been converted to use ``needs_tree_write_lock`` rather than
``needs_write_lock``. Also converted is the revert, conflicts and tree
transform modules. This provides a modest performance improvement on
metadir style trees, due to the reduce lock-acquisition, and a more
significant performance improvement on lightweight checkouts from
remote branches, where trivial operations used to pay a significant
penalty. It also provides the basis for allowing readonly checkouts.
(Robert Collins)

(Hopefully thats self explanatory :)

Next up: anonymous/readonly checkouts.

-Rob

-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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