TransportError 'Operation not permitted' while branching

[Martin Pool]

We may be getting an error because the filesystem doesn't support unix
permission bits, or doesn't allow us to set them to the value we want.
 (The name 'windata' suggests it may be fat/cifs/ntfs.)  I think there
is another bug about coping with cases where you can't pass a mode to
mkdir. (Possibly the easiest thing is to just mkdir and then chmod;
though this does leave a tiny race window on unix.)