if user is not logged in to Git the prompt for username/password

Ref. [lib/dotstrap/git.rb:18](https://github.com/mkwmms/dotstrap/blob/028fa44ef578e3fbbd9ec9b1a3910d4b2539bdb3/lib/dotstrap/git.rb#L18)

``` rb
    # FIXME: if user is not logged in to Git the prompt for username/password
    # is mangled because the threads are not synchronized
    def clone(url = @url, dir = @repo_path, repo = @repo)
      if Dir.exist?(dir)
…
```
