git manage tfinit and a little bit more.

- Add a new git manage command, tfinit, that initializes a Terraform
  backend managed by GitLab for this repository (based on the GitLab
remote).
- Better handling of cases when running in a subdirectory of the repo in
  the git module. Use git to check if a directory is a repository and to
find the top level of a repository.
- Use the top level functions' docstring for the CLI help message.
- Use the sub-parser to output better error messages.