code: public function getRootIdentifier()
      {
          if ($this->fallbackDriver) {
              return $this->fallbackDriver->getRootIdentifier();
          }
  
          if (null === $this->rootIdentifier) {
              if (! $this->getRepoData()) {
                  return $this->fallbackDriver->getRootIdentifier();
              }
  
              if ($this->vcsType !== 'git') {
                  throw new \RuntimeException(
                      $this->url.' does not appear to be a git repository, use '.
                      $this->cloneHttpsUrl.' if this is a mercurial bitbucket repository'
                  );
              }
  
              $mainBranchData = $this->getMainBranchData();
              $this->rootIdentifier = !empty($mainBranchData['name']) ? $mainBranchData['name'] : 'master';
          }
  
          return $this->rootIdentifier;
      }
docs: {@inheritDoc}