[MERGE] Enable merging PreviewTrees

[Aaron Bentley]

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Hi all,

This patch adds support for merging PreviewTrees into other trees.  This
means changing some methods to avoid using Tree.inventory, which
PreviewTree does not supply.

In the process, it introduces Tree.get_inventory_entry, which provides a
convenient way of getting single inventory entries, without actually
using Tree.inventory or Tree.iter_entries_by_dir.  This is a new
requirement for Tree implementors, but Tree provides a default for
subclasses.

Finally, it adds several caches to PreviewTree, vastly improving
performance of iter_changes.

Aaron
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