[MERGE][BUG #187169] rollback an invalid delta

[John Arbash Meinel]

This patch adds 2 updates to dirstate handling.

1) When we get into a bad state, set a flag so that the dirstate is 
considered broken, and refuse to save it. This effectively rolls back 
the state to the previously known good state.

This is partially because "update_basis_by_delta" (and 
apply_inventory_delta) are designed to allow for an inconsistent state 
in the middle of processing (rename A => B and B => A) as long as they 
are consistent at the end.

I decided to go with a separate flag (self._consistency) rather than 
another state in self._dirblock_state. Because there are a lot of places 
that blindly set self._dirblock_state = IN_MEMORY_MODIFIED, and do so 
blindly for performance reasons. I didn't want to have to change all of 
those to check if it was corrupted, and then abort somehow.

2) When detecting an invalid delta, raise a real error rather than a 
slightly customized BzrError.

I believe Robert didn't think they should be caught (which is 
reasonable), but I did want to test them, so I created a real exception 
class for them.

I'd like to see this land in 1.2. It doesn't completely fix the bug, as 
commit will still generate an invalid delta. But it does help prevent 
the dirstate from reaching an invalid state.

John
=:->
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