bug #21823 dumpFile(), preserve existing file permissions (chs2)

This PR was squashed before being merged into the 2.7 branch (closes #21823).

Discussion
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dumpFile(), preserve existing file permissions

When calling Filesystem::dumpFile() on an already existing file, its permissions are lost.

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #... <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!--highly recommended for new features-->

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Commits
-------

040a9abdbf dumpFile(), preserve existing file permissions

src/Symfony/Component/Filesystem/Filesystem.php

@@ -516,7 +516,10 @@ class Filesystem
             }
 
             $this->chmod($tmpFile, $mode);
+        } else {
+            @chmod($tmpFile, file_exists($filename) ? fileperms($filename) : 0666 & ~umask());
         }
+
         $this->rename($tmpFile, $filename, true);
     }