fixed escaping the path (it's no cmdline argument anymore)

Commit ``8b8dc1fd7017`` broke the hg support for us. Symfony Process quits while running ``composer update``:

```
Reading C:/Users/Christoph/AppData/Local/Composer/repo/https---packagist.org/provider-pimple$pimple.json from cache
  - Installing webvariants/internal-package (dev-default 3b55cca78dd0)
    Cloning 3b55cca78dd0
Executing command (CWD): hg clone "https://internal.repositories.host.org/internal-package/" "vendor/internal-package"
Executing command ("vendor/internal-package"): hg up "3b55cca78dd0" <-- note the escaped path here



  [ErrorException]
  proc_open(): CreateProcess failed, error code - 267
```

This PR fixes the escaping for the path when it's not used inside of a cmdline string.