title: git diff between two different files
body: <p>In <code>HEAD</code> (the latest commit), I have a file named <code>foo</code>. In my current working tree, I renamed it to <code>bar</code>, and also edited it.</p>
  
  <p>I want to <code>git diff</code> <code>foo</code> in <code>HEAD</code>, and <code>bar</code> in my current working tree.</p>