[MERGE] Make 'mv a b' work for already renamed directories, like it does for files (#107967)

[Robert Collins]

Robert Collins has voted approve.
Status is now: Approved
Comment:
Its a shame so much of this is basically untestable in builtins.py.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C47CE9DF0.1020705%40ukr.net%3E