title: How to restrict code checkout only to a certain user/machine combination
body: <p>How to restrict git code checkout only to a certain user/machine combination. Our projects reside in github and the user uses SSH to communicate with github for code access.</p>
  
  <p>If the private key is copied to a different machine (say in his home), its possible to download the source code on a different machine.</p>
  
  <p>We are within a LAN environment and I would prefer to restrict code access strictly to the single machine which is used by developer.</p>
  
  <p>How should I enforce this?</p>