MDL-43306 restore: fix grade item sortorder restoration

Previously, restore was blindly inserting the old sort order into the
database, causing duplicate sortorders which lead to unpredicatble
sorting results.

Now we will use the api function to add it after the original sortorder,
this means if a duplicate exists in the original location, this item
will appear after it.

backup/moodle2/restore_stepslib.php

@@ -2769,9 +2769,9 @@ protected function process_grade_item($data) {
         $gradeitem = new grade_item($data, false);
         $gradeitem->insert('restore');
 
-        //sortorder is automatically assigned when inserting. Re-instate the previous sortorder
-        $gradeitem->sortorder = $data->sortorder;
-        $gradeitem->update('restore');
+        // Reinstate the grade item close to its original sortorder, if something already exists
+        // in the original sortorer then it will be first.
+        $gradeitem->move_after_sortorder($data->sortorder);
 
         // Set mapping, saving the original category id into parentitemid
         // gradebook restore (final task) will need it to reorganise items