title: How to create a sub branch?
body: <p>My git repository has two branches, <code>master</code> and <code>bug</code>. I want to create a branch <code>bug/rep</code>. So I run :</p>
  
  <pre><code>git branch bug/rep
  </code></pre>
  
  <p>This gives the error:</p>
  
  <blockquote>
    <p>error: unable to resolve reference refs/heads/bug/rep: Not a directory
    fatal: Cannot lock the ref 'refs/heads/bug/rep'.</p>
  </blockquote>
  
  <p>How can I create a sub branch from the <code>bug</code> branch, like <code>bug/rep</code>?
  My git version is 2.3.2 on the Mac.</p>