[MERGE] Enable merging into PreviewTree

[John Arbash Meinel]

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> usage with other methods, so getting a single entry via
> iter_entries_by_dir is not uncommon.  That said, I'd be happy to make
> Tree.get_inventory_entry a Tree method.
> 
> Aaron

I think it would be great to have something like this. There are a few
cases where we just want a simple member of an inventory entry, like
"IE.revision()" and we end up having to go through
"tree.inventory[file_id].revision".

Certainly WT4 and DirStateRevisionTree can implement it without having
to build the whole Inventory which is very nice for simple probes. I
don't remember exactly when I was running into this in the past, but it
is certainly something that I remember wanting.

John
=:->
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