title: Refname 'master' is ambiguous
body: <p>I've looked at all of the other ambiguous refname questions and none of them seem to help. Why am I getting this warning?</p>
  
  <pre><code>$ git checkout master
  warning: refname 'master' is ambiguous.
  $ git show-ref master
  eef61c00da690f093063ac5a728e22fd21648104 refs/heads/master
  $ git branch -a
    checkers
    exercises
  * master
  $ git remote -v
  $ 
  </code></pre>