[MERGE] repository write groups

[Robert Collins]

This is a patch to add the concept of a write group to repository.
Hopefully it is self explanatory.

I think a useful thing to do that I haven't is to make using write
groups mandatory in the API, but I'm not sure we should do that for a
release or so - right now our code is changed, but e.g. plugin code
won't error with the new API and will still work correctly on released
repositories. On the experimental repositories it will fail because
outside a write group, writing errors.

So I can do that, but thought it would be nicer for migration to hold
off forcing people to use this, and add a requirement to the API once
we're done with the experimental changes and its in whatever final form
it will have.

This API would appear to have no users in bzrlib, which is somewhat true
- but my experimental repositories branch uses it comprehensively as-is.

-Rob
-- 
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