title: Show commit size in git log
body: <p>How can I get commit size shown in the output of <code>git log</code>?</p>
  
  <p>You may understand <em>commit size</em> as the diff between its parents and itself, or anything reasonable that tells you <em>how big</em> the commit is.</p>
  
  <p><code>git log</code> has a <code>--log-size</code> option but its the size of the log message, not the commit itself.</p>