Branch.cleanup() re-creates the checkout

The Branch.cleanup() function is implemented as:

if self.has_tree:
  self._set_up_working_tree():

However, _set_up_working_tree does:

        if os.path.exists(self.temporary_dir):
            shutil.rmtree(self.temporary_dir)
        self.tree = self.branch.create_checkout(self.temporary_dir)

In other words, Branch.cleanup() *will* delete the existing target, but then it will re download the whole thing and re-create it.

See also bug #397476, fixing that would at least make this less expensive, even if it is still wrong :)