* lisp/emacs-lisp/track-changes.el (track-changes-undo-only): New var.
(track-changes-fetch): Bind it.
(track-changes--state): New slot `undo`.
(track-changes--after): Set it.
* lisp/vc/diff-mode.el (diff--track-changes-function): Use the
new var.
* doc/lispref/text.texi (Tracking changes): Mention
`track-changes-undo-only`.
e119514ae8