undo refactoring

[phillip.lord@russet.org.uk (Phillip Lord)]

I thought to move this to emacs-devel rather than #23781 since it's not
directly relevant.


I'll think about the FIXME, that I've just added to undo.c, and see if I
can update that on master. But also, while reading the code again, I've
realised that I don't understand the reason for pre-allocating the
boundary, since when it actually used we do this:


      /* One way or another, cons nil onto the front of the undo list.  */
      if (!NILP (pending_boundary))
	{
	  /* If we have preallocated the cons cell to use here,
	     use that one.  */
	  XSETCDR (pending_boundary, BVAR (current_buffer, undo_list));
	  bset_undo_list (current_buffer, pending_boundary);
	  pending_boundary = Qnil;
	}
      else
	bset_undo_list (current_buffer,
			Fcons (Qnil, BVAR (current_buffer, undo_list)));


Why not just do

	bset_undo_list (current_buffer,
			Fcons (Qnil, BVAR (current_buffer, undo_list)));

and remove all references to pending_boundary (and therefore
`prepare_record` which is now rather badly named)? Or am I missing
something

Phil