Code Modernization: Use instanceof instead of a comparison with get_class().

Includes adjusting external libraries which are no longer maintained externally.

Props jrf.
See #50767.

git-svn-id: https://develop.svn.wordpress.org/trunk@49194 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/Text/Diff.php

@@ -276,7 +276,7 @@ class Text_Diff {
 
         $prevtype = null;
         foreach ($this->_edits as $edit) {
-            if ($prevtype == get_class($edit)) {
+            if ($edit instanceof $prevtype) {
                 trigger_error("Edit sequence is non-optimal", E_USER_ERROR);
             }
             $prevtype = get_class($edit);