title: git add Signed-off-by line using format.signoff not working
body: <p>My client git version is 1.7.0.4.</p>
  
  <p>I want to automatically add a "Signed-off-by" line for the committer at the end of the commit log message when commit a message.</p>
  
  <ol>
  <li>When I set <code>git config --global format.signoff true</code>, and run <code>git commit -m "modify something"</code>, I see no "Signed-off-by" in <code>git log</code>.</li>
  <li>If I use <code>git commit -m -s  "modify something"</code>, then "Signed-off-by" does show in <code>git log</code>.</li>
  </ol>
  
  <p>Can anyone help?</p>