Loading Group Contact Cache fails for Saved Search groups

When loading the Group Contact Cache, the SQL is generated by CRM_Contact_Form_Search_Custom_Group and modified to add the group_id into the select clause. The regex in the preg_replace() is not generic enough. It needs to account for a) whitespace before the SELECT, and b) an optional DISTINCT keyword.

The attached patch fixes this. A better solution would be to have the SQL generated properly for this case by CRM_Contact_Form_Search_Custom_Group as discussed at http://forum.civicrm.org/index.php/topic,25842.0.html