BUGFIX: handle an SVN directory deletion case

For example:

    SVN              GIT

     A                X
    / \              / \
   B   C            D   E
  / \                   |
 D   E                  C

If B is deleted in SVN, we want to find the rules mapping D and E to
Git, and delete their Git subtrees.  However, what we had missed was
that after E is deleted in Git, C still has to be rewritten.