pagestats: Use a combination of precense and link dependencies.

pagestats: Use a combination of precense and link dependencies.

This makes it more efficient.

It also fixes the same bug that I fixed in orphans recently,
that only changes to the set of displayed pages were considered (or amoung),
which missed changes to links on other pages to those.

Probably this bug was never noticed because pagestats is most often put
on a blog type page, which gets updated anyway when posts change,
and thus the tag cloud was updated.