code: public function getRootIdentifier()
      {
          if (null === $this->rootIdentifier) {
              $this->process->execute(sprintf('hg tip --template "{node}"'), $output, $this->repoDir);
              $output = $this->process->splitLines($output);
              $this->rootIdentifier = $output[0];
          }
  
          return $this->rootIdentifier;
      }
docs: {@inheritDoc}