relabel_bse: fix incorrect detection of common APs

Based on a report by Jean Kreber.

* spot/tl/relabel.cc (cut_points): Really connect children of Boolean
operators using undirected edges, not directed ones.
* tests/core/ltlrel.test: Add test cases.
* NEWS: Mention the bug.

tests/core/ltlrel.test

@@ -1,6 +1,6 @@
 #! /bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2013, 2016 Laboratoire de Recherche et Développement
+# Copyright (C) 2013, 2016, 2020 Laboratoire de Recherche et Développement
 # de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -89,3 +89,22 @@ Fp0 -> ((p1 -> (!p0 U (!p0 & p2 & p3 & X((!p0 & p3) U p4)))) U p0)
   p3   ->   !p5
   p4   ->   p4
 EOF
+
+# The next two formulas come from a report by Jens Kreber.
+t <<EOF
+!b & e U (a & b & c)
+!p0 & (p1 U (p0 & p2 & p3))
+  p0   ->   b
+  p1   ->   e
+  p2   ->   a
+  p3   ->   c
+EOF
+t <<EOF
+X!c & X(b & c & d & a U e)
+X!p0 & X(p0 & p1 & p2 & (p3 U p4))
+  p0   ->   c
+  p1   ->   b
+  p2   ->   d
+  p3   ->   a
+  p4   ->   e
+EOF