CommitBuilder tests fail with nested trees

[Aaron Bentley]

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I've updated my nested trees branch to the latest bzr.dev, and
everything passes except the CommitBuilder tests.  This is because they
call CommitBuilder.finish_inventory before the root ID has been set.

I think it makes sense to require the root ID to be set explicitly.  If
we supply a default when there's no root ID, people may misuse the API
by accident, and get the wrong root ID.

Where should this root ID setting be done?

You can see the updated nested trees work here:
http://code.aaronbentley.com/bzr/bzrrepo/nested-trees/

Aaron
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