Fatal error: require_once() [function.require]: Failed opening required '/diff.pages.inc'

This error keeps happening and I am unable to do a diff between two revisions.

Here is what I found

  if (module_exists('diff')) {
    // If Diff module is enabled, make sure it uses correct access callback
    $items['node/%node/revisions/view/%/%']['access callback'] = '_revisioning_access_node_revision';
    $items['node/%node/revisions/view/%/%']['access arguments'] = array('view revisions', 1);
  }

should actually be

  if (module_exists('diff')) {
    // If Diff module is enabled, make sure it uses correct access callback
    $items['node/%node/revisions/view']['access callback'] = '_revisioning_access_node_revision';
    $items['node/%node/revisions/view']['access arguments'] = array('view revisions', 1);
  }