[MERGE] Remove knit caches

[Aaron Bentley]

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Hi all,

Here's a patch that eliminates knit caches.  Theoretically, they could
speed up repeated reads or requests for multiple versions of a file, but
those operations are already fast, now that we have operations like
get_texts.

Knit caches don't provide a significant performance benefit for commit
operations-- the commit benchmark shows a 1% performance degradation
that may just be noise.

The log_verbose benchmark shows a 1% performance degradation, which
again is probably noise.

There aren't many operations that request multiple versions of a file.
Annotate gets multiple revisions.  It shows a 12% slowdown, but it
hasn't been optimized by using Repository.get_revisions

Even if the performance gains were more compelling, this would still be
a choice between performance and correctness, because the caches consume
an unlimited amount of memory.

Therefore, I propose that we get rid of the knit caches.  It seems to be
best to do them at a higher level, instead.

Aaron
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