track-changes.el (track-changes-undo-only): New var

* 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`.

1 files changed, 13 insertions, 0 deletions

diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index c6e3b1bdbec..c1457c49562 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -6582,6 +6582,19 @@ risk that the @var{signal} function gets triggered in the middle of it,
 because the @var{signal} is re-enabled only after @var{func} finishes.
 @end defun
 
+@defvar track-changes-undo-only
+If your code uses @code{track-changes} to perform further modifications
+to the buffer (for example, to mark the parts of the buffer that have
+been edited), then you may want to refrain from making those
+modifications when the changes are the result of an undo (which
+presumably also undoes the modifications you had applied back when the
+corresponding edit was made).
+To that end @code{track-changes-fetch} binds
+@code{track-changes-undo-only} to non-@code{nil} during calls to
+@var{func} if the changes were the result of undo.
+@end defvar
+
+
 @defun track-changes-unregister id
 This function tells the library that the tracker @var{id} does not need
 to know about buffer changes any more.  Most clients will never want to