[merge] benchmark caching

[John Arbash Meinel]

I originally tried to break the patches down into incremental updates.
But since I didn't get any responses, and I had to make another update,
I'm just submitting the whole thing to be reviewed.

summary:
  new functions on Benchmark, which can grab a cached kernel-like tree
in various states. Makes test setup time *much* more reasonable. It also
allows the cache to be preserved between runs.
  I also added support for creating the kernel-like tree somewhere other
than '.'. Which will help for certain tests that need to create more
than one tree.

new changes:
  The old code base had just done a 'bzr add', which yields a hot
hash-cache, so for tests to be consistent, we need to warm it up after
copying the tree.

John
=:->
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