[MERGE] Single lock for 'bzr diff'

[John Arbash Meinel]

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At the moment, when running 'bzr diff' (no arguments) we end up grabbing (and
releasing) far too many read locks. The reason is that 'diff_cmd_helper()'
doesn't actually lock the tree, and we haven't locked it before we entered.

Which means we actually have to re-lock the tree for each operation
(basis_tree(), which actually locks it 2 times for get_parent_ids() and for
revision_tree()) etc.

Also, because of how the timing works, we also end up with 2 different read
locks on the DirState object. I don't know how exactly this happened, but it
means that 'bzr diff' won't update the cached stat values (because it fails to
upgrade to a write lock).

The attached patch fixes a lot of this by simply adding a
tree.lock_read()/unlock() call around diff_cmd_helper. There are other ways it
could be done (have diff_cmd_helper lock the tree, and call out to
_diff_cmd_helper, etc).

This seemed easy and expedient.

I don't have a specific test for it...

John
=:->
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