[MERGE] Cache inventory.knit so we don't download it twice

[John Arbash Meinel]

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The attached patch enables caching for Knits again, but only when
explicitly requested. I don't have any giant trees lying around to test
it on.

The branch is available from here:
http://bzr.arbash-meinel.com/branches/bzr/features/cache-inventory/

But I do have an idea for spilling to disk if necessary. (Check if
branch is not local, and if not, spill to disk, else remove entries from
the cache kind of a thing).

On the plus side, it shaves off a noticeable time for branch and pull.
With this integrated into my http branch the new times are:

branch all revisions: 6m01s
pull 1500->: 4m13s
pull 1800->: 1m08s

The old times were: 6m57s, 5m05s, and 1m14s respectively.

So we shave another minute off of the big copies. And not much off of a
small one (309 revs copied). But at 1min, we probably are still hitting
a lot of latency (2 round trips for each file id, we have to read the
index before the data).

Next thing will be a format bump that inter mixes data and index.

John
=:->

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