mercurial/templates/rss/header.tmpl

keyword: use `set_clean` instead of `normal`

This is the newer, more semantic API.

Differential Revision: https://phab.mercurial-scm.org/D11153

<?xml version="1.0" encoding="{encoding}"?>
<rss version="2.0">
  <channel>
    <link>{urlbase}{url|urlescape}</link>
    <language>en-us</language>