* log-edit.el (log-edit-show-diff): New function.

* log-edit.el (log-edit-show-diff): New function.
(log-edit-mode-map, log-edit-menu): Bind it.
(log-edit-diff-function): New variable.
(log-edit): Change the 3rd param to be an alist and accept a
function that computes a diff for the files involved.

* vc.el (vc-log-edit): Add a diff function parameter to log-edit.