The heuristic that Emacs uses to add an `undo-boundary' has been

The heuristic that Emacs uses to add an `undo-boundary' has been
reworked, as it interacts poorly with functions on `post-command-hook'
or `after-change-functions'.

* lisp/simple.el: New section added.
* src/cmds.c (remove_excessive_undo_boundaries): Now in lisp.
(self_insert_command): Calls simple.el to amalgamate.
(delete_char): Calls simple.el to amalgamate.
* src/keyboard.c (last_undo_boundary): Removed.
* src/undo.c (run_undoable_change): New function.