Reduce load when rebuilding smartgroups by not getting unrequired fields

When re-loading smart groups the query retrieves the group status & group contact id although the query sensible ONLY requests the contact_id.

This is in order to display those fields on the group contacts list on the page accessed by clicking on 'contacts' from manage groups.

I am submitting a limited patch to not add on the group_contact fields when the contact_id has been explicitly requested - which is what happens from the (ideally) high-performance CRM_Contact_BAO_GroupContactCache::add function with a unit test on respect for the requested parameters.