fix collation for ancestor column in wiki, fixes #423

db/migrations/5.1.11_wiki_ancestor_collation.php

+<?php
+
+class WikiAncestorCollation extends Migration
+{
+    public function description()
+    {
+        return 'fix collation for ancestor column in wiki';
+    }
+
+    public function up()
+    {
+        $db = DBManager::get();
+
+        $db->exec('ALTER TABLE wiki CHANGE ancestor ancestor VARCHAR(255) COLLATE utf8mb4_bin DEFAULT NULL');
+    }
+}