[MERGE][0.15] Speed up 'bzr status' in xml trees

[Aaron Bentley]

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John Arbash Meinel wrote:
>> You realize that the cached-basis code is designed to be robust, right?
>>    You can put garbage there, or anything you like, and it won't fall over.

> Oh, I understand you can put garbage there. But when you are trying to
> benchmark the performance of 'bzr status' for the last 8 revisions of
> bzr, it is nice when you don't have to do a full add+commit just to
> regenerate the basis-inventory-cache.

Pull will also do it.  Anything that does wt.set_last_revision().

> Anyway, format 5 would be great for compatibility, I'm not sure how we
> would jump to format 7 when we actually need it. Since WT4 actually does
> support things like nested trees, we probably should stick with format 7.

But WT4 doesn't store basis this way, and WT3 doesn't support nested trees.

Aaron
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