code: def get_repo(task, source_env_prefix):
      
      repo = _extract_from_env_in_payload(task, source_env_prefix + '_HEAD_REPOSITORY')
      if repo is not None:
          repo = repo.rstrip('/')
      return repo
docs: Get the repo for a task.
  
      Args:
          task (ChainOfTrust or LinkOfTrust): the trust object to inspect
          source_env_prefix (str): The environment variable prefix that is used
              to get repository information.
  
      Returns:
          str: the source url.
          None: if not defined for this task.