title: Arcanist - doing an "arc diff" after a git push
body: <p>Arcanist noob here.</p>
  
  <p>I want to run "arc diff" but the problem is that I already did a "git push" to our remote git repository. Perhaps there is another arc command to see the diff even after a push? Couldn't find such a command in the Arcanist files.</p>