code: def get_backend_name(self, location):
          
          for vc_type in self._registry.values():
              path = os.path.join(location, vc_type.dirname)
              if os.path.exists(path):
                  return vc_type.name
          return None
docs: Return the name of the version control backend if found at given
          location, e.g. vcs.get_backend_name('/path/to/vcs/checkout')