[MERGE] check for duplicate file-ids in Inventory.apply_delta()

[John Arbash Meinel]

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Ian Clatworthy wrote:
> Speaking with Robert, we believe that Inventory.apply_delta() ought
> to fall over if the same file-id appears multiple times within a delta.
> In particular, deltas passed to CHKInventory.create_by_apply_delta()
> create bad deltas to pass to the lower layer is this isn't true. We
> therefore want consistency across the various APIs expecting a delta.
> 
> This patch checks the delta and adds some tests for apply_delta - none
> existed previously as best I could find.
> 
> I've also grouped the Inventory tests into read vs update groups.
> That should help when CHKInventory's land: the read tests ought to
> (be refactored to) work on both Inventory & CHKInventory while the
> update tests will only apply to Inventory given CHKInventories are
> immutable.
> 
> Ian C.
> 

BB:tweak

The only thing I would recommend is deleting the 'seen' set after you
check the stream.

John
=:->

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