Fix hgweb.filediff
The third argument to hgweb.diff should be a list, not a string.
This fixes a bug found by xorAxAx, where hgweb would also show the diff
for "README" when you ask for the diff for "Demo/cgi/README".
#header#
<title>#repo|escape# Changelog</title>
<description>#repo|escape# Changelog</description>
#entries%changelogentry#
</channel>
</rss>