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#: #file|escape# history</title>
<description>#file|escape# revision history</description>
#entries%filelogentry#
</channel>
</rss>