code: def git_pull(repo_dir, remote="origin", ref=None, update_head_ok=False):
      
      command = ['git', 'pull']
      if update_head_ok:
          command.append('--update-head-ok')
      command.append(pipes.quote(remote))
      if ref:
          command.append(ref)
      return execute_git_command(command, repo_dir=repo_dir)
docs: Do a git pull of `ref` from `remote`.