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

While investigating iter changes based commit I ran into repeated 'what
if' questions relating to the safety of partial deltas. As a result I've
written up a semi-formal model of what we do and how they work.

I'm going to follow this up with tests for the apply delta logic we have
to ensure that they deal correctly with the cases of corruption we know
about. Assuming (and I do) that they currently silently corrupt, this
may be a few days off.

-Rob