Code review comment for lp:~jameinel/bzr/1.19-bug-402778

This fixes 2 bugs found as part of bug #402778

InterDifferingSerializer was failing when the target was a stacked branch. It had 2 primary bugs

1) The inner loop was using the wrong variable, which meant that it was generating a delta for one revision, and then inserting it as the delta for a different revision.

2) It was trying to fill in parent inventories for all parents, even if some parents where ghosts in the source.

I updated the test that Andrew added. I had to add content to the revisions, because otherwise you can't tell whether the added inventories are valid. And I added a ghost parent, to make sure that we don't try to fill in ghosts that we don't have.