Result caching: minor optimization tweak [1]

While debugging something else, I noticed that `.bak` files were being used for the "PHPCS native file hash".

As non-php files in the PHPCS directories have no effect on the actual run output, we may as well skip them from being part of the hash.

src/Util/Cache.php

@@ -105,6 +105,11 @@ function ($file, $key, $iterator) {
                     return false;
                 }
 
+                // Skip non-php files.
+                if ($file->isFile() === true && substr($filename, -4) !== '.php') {
+                    return false;
+                }
+
                 $filePath = Common::realpath($file->getPathname());
                 if ($filePath === false) {
                     return false;