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Todo:

 * Fix the pack creation code in Dulwich. It doesn't generate deltas quite 
   well at the moment.

 * Switch to the new pack-based format once John's work on PackCollections 
   is finished. This should give very nice performance improvements, 
   in particular the caching of Trees.

 * Support using the cached trees in BazaarObjectStore.generate_pack_contents,
   rather than calling out to _revision_to_objects as the latter is slow
   (it uses inventories).

 * Support roundtripping