Code review comment for lp:~lifeless/bzr/bug-395556

This certainly seems to be the simple and straightforward fix. I believe a "better" fix (faster, but more complex) would be to copy the objects when we want to mutate them, rather than trying to mutate-in-place.

Considering we are trying to get away from mutate-in-place anyway (CHKInventory doesn't support apply_delta anymore, etc.)

Anyway, we might want a comment about performance if we see a regression from this. ISTR that Inventory.copy() time can easily became a dominant factor in a lot of operations. But we can certainly start here.