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)