Moved comment.list_to_root() to Bug.add_comments() with some cleanups.

Moved comment.list_to_root() to Bug.add_comments() with some cleanups.

This makes Bug.add_comment simpler.  Also makes Bug.from_xml() more
robust, since it no longer depends on the order in which the XML file
lists the comments.  The previous Bug.from_xml() would have choked on
  <be-xml>
    <bug>
      <comment>
        <uuid>B</uuid>
        <in-reply-to>A</in-reply-to>
      </comment>
      <comment>
        <uuid>A</uuid>
      </comment>
    </bug>
  </be-xml>
because when B was being added, the referenced A hadn't yet been
noticed.