MDL-37312 backup: Keep course idnumber on same site course restore

When the course idnumber is set during a same site course restore,
retain the course idnumber if there are no duplicates.

backup/moodle2/restore_stepslib.php

@@ -1472,11 +1472,14 @@ public function process_course($data) {
         $data->fullname = $fullname;
         $data->shortname= $shortname;
 
+        // Only allow the idnumber to be set if the user has permission and the idnumber is not already in use by
+        // another course on this site.
         $context = context::instance_by_id($this->task->get_contextid());
-        if (has_capability('moodle/course:changeidnumber', $context, $this->task->get_userid())) {
-            $data->idnumber = '';
+        if (!empty($data->idnumber) && has_capability('moodle/course:changeidnumber', $context, $this->task->get_userid()) &&
+                $this->task->is_samesite() && !$DB->record_exists('course', array('idnumber' => $data->idnumber))) {
+            // Do not reset idnumber.
         } else {
-            unset($data->idnumber);
+            $data->idnumber = '';
         }
 
         // Any empty value for course->hiddensections will lead to 0 (default, show collapsed).