title: Git: How to reset after merging?
body: <p>I've merged a master branch from a friend's repository into my working directory into branch_a using:</p>
  
  <pre><code>git pull my_friend master
  </code></pre>
  
  <p>I've discovered that the merged version has errors. To continue development I would like to revert to my last commit before the merge.<br>
  I tried:</p>
  
  <pre><code>git reset --hard HEAD
  </code></pre>
  
  <p>But that brought me back to the state right after the merge. (does pull command commit?!)<br>
  I also tried:</p>
  
  <pre><code>git revert HEAD
  </code></pre>
  
  <p>but received the following error:</p>
  
  <blockquote>
    <p>fatal: Commit
    2d72d8f367b987d8c16f5cb1a543a6886acdcf83
    is a merge but no -m option was given.</p>
  </blockquote>
  
  <p>What should I do?</p>