title: Create diff patch for a composer package
body: <p>I'm contributing to quite a large project with lots of submodules, and using <code>repositories</code> section of <code>composer.json</code> etc.
  Sometimes it is handy to edit files in the <code>vendor</code> directory instead of the real repository (e.g. there are some submodules with tests for the other ones).</p>
  
  <p>Then I use <code>composer update</code> and a message pops up:
  <code>Discard changes [y,n,v,d,s,?]?</code></p>
  
  <p>I can choose the option <code>d</code>, thus getting a nice diff patch I can later use for the real Git repository. Can I have such a patch in a simpler way?</p>