* diary-lib.el (diary-goto-entry-function): New variable.

* diary-lib.el (diary-goto-entry-function): New variable.
(diary-entry): Use it in the action of this button type instead of
diary-goto-entry.

* todos.el (todos-diary-goto-entry): Add item locating code from
diary-goto-entry.  Add it at the top-level to override the latter
function.
(todos-powerset): Use definition by Wolfgang Jenkner, posted at
http://lists.gnu.org/archive/html/emacs-devel/2013-06/msg00423.html.