[RFC/MERGE] quick hack for bzr log

[John Arbash Meinel]

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So it seems that "bzr log" is using "Branch.revision_history()" which
has an inefficient implementation on packs. Specifically it goes through
the Knit layer and calls get_parents() a bunch of times. Which for packs
requires a double index lookup to check for ghosts.

This rewrites it to use a Graph object.

It then does a quick hack to Pack repositories so that their
CachingParentsProvider gets to be reused as long as the lock is held.

On the Emacs repository this changes "bzr log --limit 10" time to 7s
down from 15s on my laptop. (It does it by changing the 350k lookups
down to 85k for 85k revisions.)

I don't know if it is worth merging as is, but it might be something to
think about.

This is actually based against my "revision_graph" patch which I just
submitted to PQM.

John
=:->
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