User Query: Don't blindly re-append new meta queries for capabilities.

fixes #21119.

Built from https://develop.svn.wordpress.org/trunk@28087


git-svn-id: http://core.svn.wordpress.org/trunk@27918 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -665,8 +665,10 @@ class WP_User_Query {
 				$cap_meta_query['compare'] = 'like';
 			}
 
+			if ( empty( $qv['meta_query'] ) || ! in_array( $cap_meta_query, $qv['meta_query'], true ) ) {
 				$qv['meta_query'][] = $cap_meta_query;
 			}
+		}
 
 		$meta_query = new WP_Meta_Query();
 		$meta_query->parse_query_vars( $qv );