relnotes: enable extension when running releasenotes command
Last night, when I was working on different machine, the test worked, now on a
different machine it does not. Looks like we should make sure releasenotes
extension is loaded.
Differential Revision: https://phab.mercurial-scm.org/D4425
<entry>
<title>{branch|escape}</title>
<link rel="alternate" href="{urlbase}{url|urlescape}rev/{node|short}"/>
<id>{urlbase}{url|urlescape}#branch-{node}</id>
<updated>{date|rfc3339date}</updated>
<published>{date|rfc3339date}</published>
<content type="text">{branch|strip|escape}</content>
</entry>