title: Add access to only one git repository via SSH key, not the entire host
body: <p>on a server, I want to have several git repositories. I want different users (=different SSH keys) to have access to different repositories (think of GitHub).</p>
  
  <p>I know I can add a user's SSH key to <code>authorized_keys</code> on the server. But that would give the user access to all repos. </p>
  
  <p>How can I set this up correctly? How does GitHub do it?</p>