[MERGE] deprecated EmptyTree

[Aaron Bentley]

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Robert Collins wrote:
> This patch deprecated EmptyTree, replacing all uses of it with a
> RevisionTree.

I like the concept.  I don't like using Inventory this way, because it
produces an Inventory with a tree root, and the NULL_REVISION doesn't
have a tree root.

In nested-trees, I'm using Inventory(None) to produce an inventory with
no tree root, and using Inventory() to produce an inventory whose root
is TREE_ROOT.

Aaron
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