class Rugged::SubmoduleCollection

3. checkouts the submodule
2. sets up a master branch to be tracking origin/master
1. fetches the remote
This provides a ghetto clone implementation that:

requires the target for the clone to be an empty dir.
with a workdir for the submodule. libgit2's `git_clone` however
currently libgit2's `git_submodule_add_setup` initializes a repo

def clone_submodule(repo, fetch_options)
  # the remote was just added by setup_add, no need to check presence
  repo.remotes['origin'].fetch(fetch_options)
  repo.branches.create('master','origin/master')
  repo.branches['master'].upstream = repo.branches['origin/master']
  repo.checkout_head(strategy: :force)
end