title: changing the last commit message without committing newest changes
body: <p>My ideal workflow would consist of the following steps</p>
  
  <ol>
  <li>edit the code</li>
  <li>compile</li>
  <li>git commit -a -m "commit message"</li>
  <li>start running the new binaries, tests, etc. (may take 10+ minutes)</li>
  <li>start new changes, while the binaries are still running</li>
  <li>when step # 4 is finished, edit the commit message from step # 3, <strong>without committing the changes introduced in step # 5</strong>, by adding, say, "test FOO failed"</li>
  </ol>
  
  <p>I cannot use <code>git commit -a --amend -m "new commit message"</code>, because this commits the new changes as well. I'm not sure that I want to bother with staging or branching. I wish I could just edit the commit message without committing any new changes. Is it possible?</p>