* Has bugs updating things if the bestlink of a page changes due to
  adding/removing a page. For example, if Foo/Bar links to "Baz", which is
  Foo/Baz, and Foo/Bar/Baz gets added, it will update the links in Foo/Bar
  to point to it, but will forget to update the backlinks in Foo/Baz.

* And if Foo/Bar/Baz is then removed, Foo/Bar gets a broken link,
  instead of changing back to linking to Foo/Baz.

This old bug still exists as of 031d1bf5046ab77c796477a19967e7c0c512c417.
and now this same problem also affects link dependencies.

(Some of) The buggy code is in `find_changed_links`
which doesn't detect that the link has changed in
this case, because when it looks at where the `%oldlinks` link to,
it does so after having updated state to add/remove the page.