Root: huge optimisation (and security feature) for VCS directories

If under root user, for VCS directories we do not collect anymore the
VCS status information. Before it was collected (even if
LP_ENABLE_VCS_ROOT=0, which is the default) but just not displayed.
This is a huge optimisation for VCS directories, and also a security
feature, as our shell code is absolutely not resistant to specially
crafted VCS content.