Move limbo marker dereferencing logs while not doing anything, it's confusing

[Facundo Batista]

In the following code...

                # both parents can be the same marker at the same time
                if old_parent == marker or new_parent == marker:
                    if old_parent == marker:
                        old_parent = value
                    if new_parent == marker:
                        new_parent = value
                log_debug("dereference ok move limbo: share=%r node=%r "
                          "marker=%r old_parent=%r new_parent=%r",
                          share, node, marker, old_parent, new_parent)
                self.move_limbo[k] = (old_parent, new_parent, new_name,
                                      path_from, path_to)

... the logging and the dict overwritting should be done only if something really changed