If a page inlines some other page (such as this page by the bugs page), 
and the page is removed (such as by this page being linked to bugs/done),
the inlining page is not updated to remove it.

This only happens if the page is removed from the inlined pagespec due to
a tag changing; the problem is that once the tag is changed, ikiwiki does
not know that the page used to match before.

To fix, seems I would need to record the actual list of pages that are
currently included on an inline page, and do a comparison to see if any
have changed. At first I thought, why not just add them to the dependencies
explicitly, but that fails because the dependencies pagespec fails to match
when a negated expression like "!tag(bugs/done)" is matched.

So, quick fixes aside, what's the generic mechanism here that a plugin can
use to let ikiwiki know that a page should be updated if some other page
stops matching its dependencies pagespec?