hgweb: add branches RSS and Atom feeds
There were no RSS nor Atom feeds for the branches page. Different hgweb
templates linked to different feeds on their branches page (some linked to the
tags feed, some to the log feed and some to the unexisting branches feed).
{header}
<title>{repo|escape}: branches</title>
<description>{repo|escape} branch history</description>
{entries%branchentry}
</channel>
</rss>