[MERGE] Reorganize TreeTransform inventory delta creation

[Aaron Bentley]

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Robert Collins wrote:
> If self._removed_contents etc are dictionaries, you may find a
> performance improvement by doing
> changed_kind.intersection_update(frozenset(self._new_contents))
> etc

Thanks for the tip.  In this case, _inventory_altered doesn't even take
1% of runtime, so it's not worth it.

Aaron
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