[MERGE] Comparison cache to speed up diff

[Aaron Bentley]

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Robert Collins wrote:
> Robert Collins has voted reject.
> Status is now: Vetoed
> Comment:
> Seems extremely special cased

Not at all.  Have you read my diff performance assessment?  Getting fast
comparisons of working tree texts to basis texts was a prerequisite to
getting fast comparisons of working tree texts to arbitrary historical
texts.

> and likely not needed. I think we should
> reevaluate this in future after we have sped up sequence matcher and so on.

Agreed.

Aaron
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