title: git: submodule tracking current branch
body: <p>According to the <a href="https://git-scm.com/docs/git-submodule#git-submodule---branch" rel="nofollow noreferrer">git documentation</a>, I should be able to run this command:</p>
  
  <pre><code>git submodule add -b . https://my/repo
  </code></pre>
  
  <p>And have a sub-module added, which will track head of the current branch of the super project.</p>
  
  <blockquote>
    <p>Branch of repository to add as submodule. The name of the branch is recorded as submodule..branch in .gitmodules for update --remote. A special value of . is used to indicate that the name of the branch in the submodule should be the same name as the current branch in the current repository.</p>
  </blockquote>
  
  <p>But when I do this, I get this error:</p>
  
  <blockquote>
    <p>fatal: 'origin/.' is not a commit and a branch '.' cannot be created from it</p>
  </blockquote>
  
  <p>I'm running git 2.21.  Have I read the instructions completely wrong?</p>