Adding a binding for 'undo-redo'

[Dmitry Gutov <dgutov@yandex.ru>]

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Hi all,

This discussion has been living in a subthread, but I think it deserves 
a separate one, for all to see.

The main technical hurdle to adding this binding was the lack of a good 
binding, but Richard suggested C-? and C-M-_, which should cover both 
GUI Emacs and tty. This change doesn't change the existing 'undo' 
binding, so it's not going to break anybody's workflow. But it adds a 
key combination we can recommend to the users who are accustomed to 
separate 'undo' and 'redo' commands.

WDYT? Should we add this to Emacs 28?

A follow-up patch could add 'undo-redo' to cua-mode as well.

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diff --git a/etc/NEWS b/etc/NEWS
index 21cc3c50cd..6b1e74eddd 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -957,7 +957,11 @@ for CJK text mixed with Latin text.
 +++
 ** New command 'undo-redo'.
 It undoes previous undo commands, but doesn't record itself as an
-undoable command.
+undoable command. It is bound to 'C-?' and 'C-M-_', the first binding
+works well in graphical mode, and the second one is easy to hit on tty.
+
+For full conventional undo/redo behavior, you can also customize the
+user option 'undo-no-redo' to t.
 
 +++
 ** New commands 'copy-matching-lines' and 'kill-matching-lines'.
diff --git a/lisp/bindings.el b/lisp/bindings.el
index 343f1ba0fa..9a3505058c 100644
--- a/lisp/bindings.el
+++ b/lisp/bindings.el
@@ -994,6 +994,9 @@ undo-repeat-map
   "Keymap to repeat undo key sequences `C-x u u'.  Used in `repeat-mode'.")
 (put 'undo 'repeat-map 'undo-repeat-map)
 
+(define-key global-map (kbd "C-?") 'undo-redo)
+(define-key global-map [?\C-\M-_] 'undo-redo)
+
 (define-key esc-map "!" 'shell-command)
 (define-key esc-map "|" 'shell-command-on-region)
 (define-key esc-map "&" 'async-shell-command)
diff --git a/lisp/simple.el b/lisp/simple.el
index dd480396f6..7eba33116d 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -2942,8 +2942,9 @@ undo-equiv-table
 (defvar undo-in-region nil
   "Non-nil if `pending-undo-list' is not just a tail of `buffer-undo-list'.")
 
-(defvar undo-no-redo nil
-  "If t, `undo' doesn't go through redo entries.")
+(defcustom undo-no-redo nil
+  "If t, `undo' doesn't go through redo entries."
+  :type 'boolean)
 
 (defvar pending-undo-list nil
   "Within a run of consecutive undo commands, list remaining to be undone.