commit 5cf79a19dac5187b6e8f4f6c3798d0f348eff89c
Author: Campbell Barton <ideasman42@gmail.com>
Date:   Fri Oct 29 12:33:29 2021 +1100

    Add a version of atomic-change-group that amalgamates undo steps

diff --git a/lisp/subr.el b/lisp/subr.el
index 39676249cd..4d2ca0b953 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3490,15 +3490,8 @@ y-or-n-p
 
 ;;; Atomic change groups.
 
-(defmacro atomic-change-group (&rest body)
-  "Like `progn' but perform BODY as an atomic change group.
-This means that if BODY exits abnormally,
-all of its changes to the current buffer are undone.
-This works regardless of whether undo is enabled in the buffer.
-
-This mechanism is transparent to ordinary use of undo;
-if undo is enabled in the buffer and BODY succeeds, the
-user can undo the change normally."
+(defmacro atomic-change-group-1 (undo-amalgamate &rest body)
+  "Implementation of `atomic-change-group' & `atomic-change-group-amalgamate'."
   (declare (indent 0) (debug t))
   (let ((handle (make-symbol "--change-group-handle--"))
 	(success (make-symbol "--change-group-success--")))
@@ -3519,9 +3512,28 @@ atomic-change-group
 	 ;; Either of these functions will disable undo
 	 ;; if it was disabled before.
 	 (if ,success
-	     (accept-change-group ,handle)
+             (progn
+	       (accept-change-group ,handle)
+               (when ,undo-amalgamate
+                 (undo-amalgamate-change-group ,handle)))
 	   (cancel-change-group ,handle))))))
 
+(defmacro atomic-change-group (&rest body)
+  "Like `progn' but perform BODY as an atomic change group.
+This means that if BODY exits abnormally,
+all of its changes to the current buffer are undone.
+This works regardless of whether undo is enabled in the buffer.
+
+This mechanism is transparent to ordinary use of undo;
+if undo is enabled in the buffer and BODY succeeds, the
+user can undo the change normally."
+  `(atomic-change-group-1 nil ,@body))
+
+(defmacro atomic-change-group-undo-amalgamate (&rest body)
+  "A version of  `atomic-change-group' that amalgamates
+the change groups undo data."
+  `(atomic-change-group-1 t ,@body))
+
 (defun prepare-change-group (&optional buffer)
   "Return a handle for the current buffer's state, for a change group.
 If you specify BUFFER, make a handle for BUFFER's state instead.