FileSystem::makePathRelative is broken when resolving less than two path elements

Running:

``` php
$f = new Symfony\Component\Filesystem\Filesystem();
echo $f->makePathRelative('/home/user', '/var'), "\n";
```

gives:

```
home/user
```

Expected is: `../home/user`.

However,

``` php
$f = new Symfony\Component\Filesystem\Filesystem();
echo $f->makePathRelative('/home/user/drm', '/usr/local'), "\n";
```

gives the expected `../../home/user/drm/`
