[RFC] Faster load of full inventory for development6-rich-root?

[Ian Clatworthy]

Partial inventory processing is now much faster in
development6-rich-root than previous formats. In general, we want to
avoid processing a full inventory, but there are times when we need to
and, unfortunately, that's terribly slow right now. :-(

Benchmarking on OOo with 75k files shows two cases in particular:

* 97% of bzr send is taken up in walking 2 inventories: one to find
  what to include in the bundle and one to calculate the testament_sha1
  value of the revision to merge.

* 99% of "bzr ls -1 --recursive" is iter_entries_by_dir().