More optimization

[erkyrath]

We could shave another 1.4 seconds (25% or so) on Index-only updates if we skipped the date_X.html pages. (They have no metadata or description lists, so they don't need to be touched if only Index files have changed. In particular, the every-file list at date.html is enormous.)

Index-only updates are unfortunately not easy to detect.

[erkyrath]

Now we have datev_X.html pages, which do depend on metadata. So this idea has reduced impact even if I figured out how to do it.