lock free dirstate

[Robert Collins]

I plan to do an experiment this week on an OSLock free dirstate - this
should work more robustly on NFS, AFP etc. I've tagged the bugs that
stood out as being fixed by this as 'dirstate2' in launchpad.

The design I plan to use is the one that I've spoken about before:
 - write a collection of dirstate files named by hash (like we do
in .bzr/repository)
 - have old dirstate files attempt to rm but handle them being open by
other processes by leaving them behind
 - have a near-atomic pointer file in the root that is updated by the
standard foo.tmp, mv foo foo.old, mv foo.tmp foo, rm foo.old code

I figure this should be pretty straight forward to hook in. It will mean
a new tree format, but if successful will allow 'bzr diff' from a commit
editor to work and so forth.

If there are any objections to this design, please speak up now!

-Rob
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