[FileSystem] makePathRelative incorrect output when $startPath has ".." in it

| Q                | A
| ---------------- | -----
| Bug report?      | yes
| Feature request? | no
| BC Break report? | no
| RFC?             | no
| Symfony version  | 3.2.0

```php
$endPath = 'C:\web\js\app.js';
$startPath = 'C:\web\css\..\\';
$filesystem->makePathRelative($endPath, $startPath);
```
Expected output is `js\app.js` but this snippet outputs `../../js/app.js/`.
