title: git branch -d gives warning
body: <p>Just want to get a better understanding of the warning message after I deleted a local branch</p>
  
  <blockquote>
    <p>warning: deleting branch <code>'old_branch'</code> that has been merged to
             <code>'refs/remotes/origin/old_branch'</code>, but not yet merged to <code>HEAD</code>.</p>
  </blockquote>