Include querystring as part of cached filename for repo summary page

When adding support for the h parameter to the summary page (passing current
branch between pages), the builtin cache returned basically random results
for summary page since the cached filename didn't honour the querystring.

This fixes the issue for now, but someday it might be worthwhile to generate
'canonical' filenames in the cache for all pages, i.e. something a bit more
clever than just including the querystring.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>