title: How to create patch for staged files with Magit
body: <p>How could I create patch for staged files with Magit?</p>
  
  <p>I tried "W p", when prompt "Format range or commit(master)", I am not sure what should I put there, I tried HEAD..master, but it doesn't create any patch. </p>
  
  <p>I tried command line: "git diff --cached > my.patch" create what I expected. </p>
  
  <p>How could I create the same patch with Magit?</p>