[MERGE] Optimize single-file diff/revert/etc

[Aaron Bentley]

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John Arbash Meinel wrote:
> Aaron Bentley wrote:
> 
> What I would love to see is the ability to push this sort of the all the
> way down to the XML parsing layer, so it wouldn't have to create
> InventoryEntry objects for anything but the necessary paths.... But now
> is not the time for that. :)

Well, this is just a stopgap until we have distate trees, anyhow.  With
a custom implementation of _iter_changes, these operations should scream.

> Have you done any performance testing to see if this actually makes a
> difference? It seems like it should, but I wonder if it is a large
> difference.

I've done a little testing.  On my biggest tree, it dropped single-file
diff from 15s to 10s.

> I'm going to do a little performance testing on the Mozilla source tree
> (not a full conversion, just a single snapshot), I'll let you know what
> I find.

Sounds good.

Aaron
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