If a change to a page is previewed, but not saved, `%pagestate` and
`%wikistate` can be changed, and saved. Actually, it's not limited to
those. Seems that spurious dependencies can be added, though existing
dependencies will at least not be removed.

It calls saveindex to record state about files created on disk for the
preview. Those files will expire later. However, saveindex also
saves other state changes.

Seems like it needs to isolate all state changes when previewing... ugh.
--[[Joey]] 

[[done]]
