code: func (c Checkout) Filter(whiteset stringset.Set) Checkout {
  	newCheckout := make(Checkout)
  	for repo, commit := range c {
  		if whiteset.Has(repo) {
  			newCheckout[repo] = commit
  		}
  	}
  	return newCheckout
  }
docs: // Filter filters out repositories from the Checkout which are not in the whitelist
  // and returns a new Checkout.