title: Tagging a subrepository only
body: <p>I have a Mercurial main-repository with one sub-repository, like this:</p>
  
  <pre><code>Main
    .hg
    .hgsub
    .hgsubstate
    &lt;some_regular_files_and_subdirs&gt;
    Sub
      .hg
      &lt;some_regular_files_and_subdirs&gt;
  </code></pre>
  
  <p>And I wish to tag <code>Sub</code>, only, while keeping <code>Main/.hgsubstate</code> up-to-date (i.e. point to the changeset of <code>Sub</code> resulting from the tagging process).<br>
  I don't want to tag the main-repository.<br>
  Is that possible without tagging from within <code>Sub</code> and do some magic to get <code>Main/.hgsubstate</code> up to date?</p>