title: Why must I force push after changing a commit message?
body: <p>I read <a href="https://stackoverflow.com/a/179147/6850771">this</a> about how to amend commit messages. The accepted answer says:</p>
  
  <blockquote>
    <p>If you've already pushed your commit up to your remote branch, then you'll need to force push the commit with <code>git push &lt;remote&gt; &lt;branch&gt; --force</code>.</p>
  </blockquote>
  
  <p>It's my understanding (also from the accepted answer) that <code>git push --force</code> will overwrite all data on the remote branch with the local one.</p>
  
  <p>Why is force-pushing after changing a commit message necessary? What happens if I amend a commit message and try to push without <code>-f</code> or <code>--force</code>?</p>