Fix a bug that prevented matching deleted comments, and so did not update pages that...

Fix a bug that prevented matching deleted comments, and so did not update pages that had contained them.

Problem is that by the time rendering calls render_dependent, %pagesources
has had deleted files removed from it. So match_comment's lookup of
files in there to see if they had the _comment extension failed.

I had to introduce a hash that temporarily holds filenames of deleted pages
to fix this.

Note that unlike comment(), internal() had avoided this pitfall by being
defined to match both internal and non-internal pages.