MDL-61163 mod_wiki: guests should be able to view frontpage modules

When wiki is added to the frontpage and the guest role has capability to view
wiki, users should not be required to log in.

mod/wiki/diff.php

@@ -66,7 +66,7 @@ if ($compare >= $comparewith) {
     print_error('cannotcomparenewerversion', 'wiki');
 }
 
-require_login($course, true, $cm);
+require_course_login($course, true, $cm);
 
 if (!wiki_user_can_view($subwiki, $wiki)) {
     print_error('cannotviewpage', 'wiki');