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

While my work on validating inventory deltas is not complete, this
particular patch is self contained and already has a number of
significant bugs fixed. In particular it catches silent corruption of
dirstate basis trees by malformed commits.

I'd like this reviewed at this point as landing it will may well improve
the user experience by turning fail-later experiences into fail-early,
making some other bugs easier to diagnose and fix. I have yet to check
that there aren't corruptions being caused unwittingly by the test suite
- however I'll do that concurrently with review.

-Rob

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