Fix bug #8782.

Fix bug #8782.

We need to invalidate the BibTeX cache when undoing or redoing. I do
not like having to do it for every undo or redo. We should only have
to do it if we restored or deleted an InsetBibTeX. But there is no
way, so far as I can see, to do it that way. I tried.

(cherry picked from commit 02847641a8b97a57d1866120bb446427e0bda7a4)