Diff should track the variables that it defines

Create diff.install & paste in

/**
 * @file
 * Provides uninstallation functions.
 */

/**
 * Implements hook_uninstall().
 */
function diff_uninstall() {
  // Unset diff variables.
  $node_types = array_keys(node_type_get_types());
  foreach ($node_types as $node_type) {
    variable_del('enable_revisions_page_' . $node_type);
    variable_del('show_diff_inline_' . $node_type);
    variable_del('show_preview_changes_' . $node_type);
  }
}

And also there is no tracking of these settings if the machine name is altered. This one is for the main module file.

/**
 * Implements hook_node_type_update().
 *
 * This tracks the diff settings in case the node content type is renamed.
 */
function diff_node_type_update($info) {
  if (!empty($info->old_type) && $info->old_type != $info->type) {
    $type_variables = array(
      'show_preview_changes',
      'enable_revisions_page',
      'show_diff_inline',
    );
    foreach ($type_variables as $prefix) {
      $setting = variable_get($prefix . '_' . $info->old_type, FALSE);
      if ($setting !== FALSE) {
        variable_del($prefix . '_' . $info->old_type);
        variable_set($prefix . '_' . $info->type, $setting);
      }
    }
  }
}

I missed the content type delete hook :)

Fixing tags

This still applies to the 6.x branches and 7.x-2.x branch, but has been fixed in 7.x-3.x branch.

Marking this as fixed, as it is committed to the latest dev branch.

Please reopen this issue if this change introduced any bugs.

Follow-ups should be added in separate issues.