MDL-49332 gradebook: Raise time limit if we get into regrade

Because all regrading should (and does) start from this point, it
is safe and best to call it just once, from this point.

lib/gradelib.php

@@ -1014,6 +1014,8 @@ function grade_recover_history_grades($userid, $courseid) {
  * @return bool true if ok, array of errors if problems found. Grade item id => error message
  */
 function grade_regrade_final_grades($courseid, $userid=null, $updated_item=null) {
+    // This may take a very long time.
+    \core_php_time_limit::raise();
 
     $course_item = grade_item::fetch_course_item($courseid);