Code review comment for lp://staging/~mwhudson/loggerhead/clean-up-inventory-view

Third time lucky?

Hi Martin (probably :),

This branch cleans up the inventory view considerably, showing the way
I'd like to go with loggerhead over the next few days. It's not got as
far as removing all use of history.py, at all, but it moves a chunk of
bzrlib usage closer to the action.

I also cleaned up (well, added) the input validation and added a fair
number of tests.

Cheers,
mwh