title: Show merge parents in git log when using pretty format string
body: <p>Git log <a href="https://git-scm.com/docs/git-log" rel="nofollow">documentation</a> says:</p>
  
  <blockquote>
    <p>If the commit is a merge, and if the pretty-format is not oneline, email or raw, an additional line is inserted before the Author: line. This line begins with "Merge: " and the sha1s of ancestral commits are printed, separated by spaces. </p>
  </blockquote>
  
  <p>Buy I'm using format.pretty in my git global config, and I don't see the "Merge line".</p>
  
  <p>I could emulate it with the %p (or %P) parameters shown in the <a href="https://git-scm.com/docs/git-log" rel="nofollow">git-log documentation</a>:</p>
  
  <blockquote>
    <p>%P: parent hashes</p>
    
    <p>%p: abbreviated parent hashes</p>
  </blockquote>
  
  <p>But that shows an empty "Merge:" line if the commit is not a merge commit.</p>
  
  <p>Is there a way to emulate the standard log behavior about merge commits parents while using a pretty format string?</p>