Fixer: improve performance of generateDiff()

Similar to 351 and 354, this is a change intended to make the test suite faster, in particular when running on Windows, as a quality of life improvement for contributing developers.

However, this change has the added benefit that the performance improvement will also be noticeable for users running PHPCS with the `diff` report, which also uses the `Fixer::generateDiff()` method.

On its own, this change makes running the test suite ~50% faster on Windows.
In combination with the other two changes, the difference is smaller, but still ~10%.

src/Fixer.php

@@ -12,6 +12,7 @@
 
 namespace PHP_CodeSniffer;
 
+use PHP_CodeSniffer\Exceptions\RuntimeException;
 use PHP_CodeSniffer\Files\File;
 use PHP_CodeSniffer\Util\Common;
 
@@ -226,6 +227,8 @@ public function fixFile()
      * @param boolean $colors   Print coloured output or not.
      *
      * @return string
+     *
+     * @throws \PHP_CodeSniffer\Exceptions\RuntimeException when the diff command fails.
      */
     public function generateDiff($filePath=null, $colors=true)
     {
@@ -246,19 +249,56 @@ public function generateDiff($filePath=null, $colors=true)
         $fixedFile = fopen($tempName, 'w');
         fwrite($fixedFile, $contents);
 
-        // We must use something like shell_exec() because whitespace at the end
+        // We must use something like shell_exec() or proc_open() because whitespace at the end
         // of lines is critical to diff files.
+        // Using proc_open() instead of shell_exec improves performance on Windows significantly,
+        // while the results are the same (though more code is needed to get the results).
+        // This is specifically due to proc_open allowing to set the "bypass_shell" option.
         $filename = escapeshellarg($filename);
         $cmd      = "diff -u -L$filename -LPHP_CodeSniffer $filename \"$tempName\"";
 
-        $diff = shell_exec($cmd);
+        // Stream 0 = STDIN, 1 = STDOUT, 2 = STDERR.
+        $descriptorspec = [
+            0 => [
+                'pipe',
+                'r',
+            ],
+            1 => [
+                'pipe',
+                'w',
+            ],
+            2 => [
+                'pipe',
+                'w',
+            ],
+        ];
+
+        $options = null;
+        if (stripos(PHP_OS, 'WIN') === 0) {
+            $options = ['bypass_shell' => true];
+        }
+
+        $process = proc_open($cmd, $descriptorspec, $pipes, $cwd, null, $options);
+        if (is_resource($process) === false) {
+            throw new RuntimeException('Could not obtain a resource to execute the diff command.');
+        }
+
+        // We don't need these.
+        fclose($pipes[0]);
+        fclose($pipes[2]);
+
+        // Stdout will contain the actual diff.
+        $diff = stream_get_contents($pipes[1]);
+        fclose($pipes[1]);
+
+        proc_close($process);
 
         fclose($fixedFile);
         if (is_file($tempName) === true) {
             unlink($tempName);
         }
 
-        if ($diff === null) {
+        if ($diff === false || $diff === '') {
             return '';
         }