* lisp/vc/log-edit.el (log-edit-diff-fileset): New function.
* lisp/vc/vc-dispatcher.el (vc-log-edit): Set log-edit-diff-function to
log-edit-diff-fileset instead of vc-diff.
* lisp/vc/vc.el (vc-diff): New optional arg ‘fileset’.
If non-nil, use instead of vc-deduce-fileset.