code: def cache_key(self, repo: str, branch: str, task: Task, git_repo: Repo) -> str:
          
          return "{repo}_{branch}_{hash}_{task}".format(repo=self.repo_id(repo),
                                                        branch=branch,
                                                        hash=self.current_git_hash(repo, branch, git_repo),
                                                        task=task.hash)
docs: Returns the key used for storing results in cache.