def get_checkout_command(self, revision, clean=False):
raise NotImplementedError()<EOL>
Get the command to update the working tree of the local repository.

:param revision: The revision to check out (a string,
                 defaults to :attr:`default_revision`).
:param clean: :data:`True` to discard changes in the working tree,
              :data:`False` otherwise.

This method needs to be implemented by subclasses.