[MERGE REVIEW] Use InventoryDirectory to represent tree root

[Aaron Bentley]

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John Arbash Meinel wrote:
> So you are saying that the 'read()' serializers can create a tree of
> this type. Because it is possible for the XML to be missing the root
> entry, right?

No, the only way I know that such an inventory can be constructed is by
doing Inventory(root_id=None).

> What did you need to add that you wanted to create a real root node in
> the XML? Or was it just to remove some special cases?

We're going to want to treat root as a normal directory as much as
possible.  We may hide it in the UI, but it will be necessary to move
the root directory around, in order to support nested by-value trees.

So we need it to have a revision property, and everything else a normal
directory entry has, with the possible exceptions of parent and name.

Aaron
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