Code review comment for lp:~spiv/bzr/inventory-delta

Ok, third time lucky! :)

Some notable changes since the last review:

  - Uses an inventory-deltas substream rather than inventing a new content factory;
  - Adds a few debug flags to control whether this code path is used or not;
  - Fixes some bugs relating to rich roots and to deletes in inventory_delta.py!

I was surprised to realise that, despite the expectations of inventory_delta.py, non-rich-root repos can have roots with IDs other than 'TREE_ROOT'. What they can't have is a root entry with a revision that doesn't match the inventory's revision.

This beats InterDifferingSerializer for a push of bzr.dev -r2000 from 1.9->2a over localhost HPSS by about 3x (9.5min vs. ~30min, although not on a totally quiescent laptop). LocalTransport push with IDS is about 10x faster than that, though.