title: Given two directory trees, how can I find out which files differ by content?
body: <p>If I want find the differences between two directory trees, I usually just execute:</p>
  
  <pre><code>diff -r dir1/ dir2/
  </code></pre>
  
  <p>This outputs exactly what the differences are between corresponding files.  I'm interested in just getting a list of corresponding files whose content differs.  I assumed that this would simply be a matter of passing a command line option to <code>diff</code>, but I couldn't find anything on the man page.</p>
  
  <p>Any suggestions?</p>