title: Fatal error on "git add submodule"
body: <p>I get the following error when I try adding a submodule to my repository:</p>
  
  <blockquote>
    <p>fatal: pathspec 'submodule' did not match any files</p>
  </blockquote>
  
  <p>I had some issues with my submodules so I had to remove them all (removed them from <code>.gitmodules</code>, <code>.git/config</code>, cleaned the index using <code>rm --cached submodule_path</code>, even did <code>git reset --mixed</code>).</p>
  
  <p>But now I can't seem to be able to add any submodule at all! I looked through the whole .git folder in search for references to my previous submodules and couldn't find anything. Have I run out of options?</p>
  
  <p>(now that I think of it I might have deleted a repository to which was referenced a submodule at some point, but that submodule is now gone so why would this be a problem?)</p>