fix as root, we only got the LP_MARK_DISABLED

cf2ff289c8755019bba5dff271ae3d03a784fc5e introduced kind of a bug.

```LP_DISABLED_VCS_PATH``` being defined to ```/``` when we're root, we always got the ```LP_MARK_DISABLED```, wherever we were, at the end of the root prompt instead of the classic root mark (which, for me, is ```#```).