org-depend: Fix blocking logic

org-depend: Fix blocking logic

* contrib/lisp/org-depend.el (org-depend-block-todo): Fix blocking logic.

Paul Sexton writes:

> The value returned by org-depend-block-todo determines whether an item
> is considered "blocked", and thus whether it is shown dimmed (because
> this function is added to org-blocker-hook).
>
> o-d-b-t only returns true in a very limited set of conditions, so very
> often items which should show up as blocked, do not. To be honest I
> don't remember the exact details, but it did not work before I applied
> the patch, and has worked correctly ever since.