docs: def down(context, repo_url):
      """(download) Synchronise remote repo to local repo.
  
      If repo_url is given, then clone from remote URL.
      """
      if repo_url == '':
          context.obj.find_repo_type()
          if context.obj.vc_name == 'git':
              context.obj.call(['git', 'pull'])
          elif context.obj.vc_name == 'hg':
              context.obj.call(['hg', 'pull', '-u'])
      else:
          context.obj.call([context.obj.vc_name, 'clone', repo_url])