title: git diff with author filter
body: <p>I have a series of commits by different authors and I would like to see a <code>git dff</code> output between 2 commits but only considering the commits by one of the authors, something like, something like --author in git log.</p>
  
  <p>I am interested in the final summary diff and not the diffs of the individual commits.</p>
  
  <p>Is there a git trick for that?</p>