hgk: don't use mutable default argument value
Caught by pylint.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US">
<head>
<link rel="icon" href="{staticurl|urlescape}hgicon.png" type="image/png" />
<meta name="robots" content="index, nofollow" />
<link rel="stylesheet" href="{staticurl|urlescape}style-paper.css" type="text/css" />
<link rel="stylesheet" href="{staticurl|urlescape}style-extra-coal.css" type="text/css" />
<script type="text/javascript" src="{staticurl|urlescape}mercurial.js"></script>