title: git submodule add error: does not have a commit checked out
body: <p>I create a new git repository with the help of bitbucket called <em>confusionmatrix</em>. Now, I'm trying in another git repository (called <em>workspace</em>) add the <em>confusionmatrix</em> repository as submodule like this:</p>
  
  <pre><code>git submodule add https://....@bitbucket.org/..../confusionmatrix.git
  </code></pre>
  
  <p>But I'm getting the following error:</p>
  
  <blockquote>
    <p>'confusionmatrix' does not have a commit checked out</p>
  </blockquote>
  
  <p>I already made this with other git repositories to the same repository "workspace" and worked well.</p>
  
  <p>What I'm doing wrong?</p>