mercurial/templates/rss/header.tmpl

tests: allow more detailed error message from re.compile

test-grep.t now passes on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D4240

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