Fix creation of bare repository

If the repository is bare, it shouldn't checkout any branch, as this causes the following error:

```
err: /Stage[main]/Project/Git::Repo[project]/Exec[git_project_co_branch]/returns: change from notrun to 0 failed: /usr/bin/git checkout  returned 128 instead of one of [0] at .../puppet-git/manifests/repo.pp:90
```