New Diff library in core

PhpWiki's Diff classes, previously provided by the Diff module for Drupal 7, have been added to Drupal 8 core.

Note: For now, the Diff classes have been dumped as-is into /core/lib/Drupal/Component/Diff/DiffEngine.php. The classes will soon be refactored into a proper PSR-4-compatible PHP component, which can be leveraged by PHP frameworks and applications outside of Drupal. Until then, the Diff classes can not be autoloaded and need to be included manually. Usage documentation will be added to this change notice once the code has been refactored.