minor #24139 [Fabbot] Do not run php-cs-fixer if there are no change in src/ (lyrixx)

This PR was merged into the 2.7 branch.

Discussion
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[Fabbot] Do not run php-cs-fixer if there are no change in src/

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | kind of
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/24136#issuecomment-328057076
| License       | MIT
| Doc PR        | -

Commits
-------

3eb79e5197 [Fabbot] Do not run php-cs-fixer if there are no change in src/

.php_cs.dist

@@ -1,5 +1,9 @@
 <?php
 
+if (!file_exists(__DIR__.'/src')) {
+    exit(0);
+}
+
 return PhpCsFixer\Config::create()
     ->setRules(array(
         '@Symfony' => true,