title: Can't add git submodule when specified as a relative path
body: <p>I'm trying to add a submodule to my git repo, and I'm getting this error in return:  </p>
  
  <pre><code>remote origin does not have a url defined in .git/config
  </code></pre>
  
  <p>any ideas about what this might be? I tried googling for it but only one vague link comes up.</p>
  
  <p>I'm doing this:  </p>
  
  <pre><code>git submodule add ../extern/Lib1 lib  
  </code></pre>
  
  <p>I'm expecting this to create a submodule <code>lib/Lib1</code><br>
  I'm aware that this will only create a reference and that I then have to update/init (not crystal clear on this part, haven't gotten that far; I'm just learning the submodule command).</p>