Copy alist instead of deep copy on Grob clone

lily/lily-guile.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 1998--2020 Jan Nieuwenhuizen <janneke@gnu.org>
   Han-Wen Nienhuys <hanwen@xs4all.nl>
 
   LilyPond is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.
@@ skipping 718 matching lines @@
   SCM b;
   SCM c;
   SCM d;
 };
 
 /* inserts at front, removing duplicates */
 SCM ly_assoc_prepend_x (SCM alist, SCM key, SCM val)
 {
   return scm_acons (key, val, scm_assoc_remove_x (alist, key));
 }
+
+SCM
+ly_alist_copy (SCM alist)
+{
+  SCM l = SCM_EOL;
+  SCM *tail = &l;
+  for (; scm_is_pair (alist); alist = scm_cdr (alist))
+    {
+      *tail = scm_acons (scm_caar (alist), scm_cdar (alist), *tail);
+      tail = SCM_CDRLOC (*tail);
+    }
+  return l;
+}