title: GitLab CI: git pull instead git clone
body: <p>I have git directory on server. 
  Every push in Master branch i want to do git pull in this directory.
  I try to do that with gitlab-ci-multi-runner.
  I made <em>shell</em> runner, but by default he do repository clone, and then call my commands. </p>
  
  <p>How to change git clone on git pull?</p>