Fix #54: Acquire problem vars on clingo init.

* A clingo propagator might add watches over newly added problem
  variables during initialization. So ensure that all problem
  variables are known to the solver before calling
  ClingoPropagatorInit::init().

src/clingo.cpp

@@ -187,6 +187,7 @@ void ClingoPropagator::destroy(Solver* s, bool detach) {
 bool ClingoPropagator::init(Solver& s) {
 	POTASSCO_REQUIRE(s.decisionLevel() == 0 && prop_ <= trail_.size(), "Invalid init");
 	Control ctrl(*this, s, state_init);
+	s.acquireProblemVars();
 	init_  = call_->init(init_, ctrl);
 	front_ = call_->checkMode() == ClingoPropagatorCheck_t::Fixpoint ? -1 : INT32_MAX;
 	return true;

tests/facade_test.cpp

@@ -2008,6 +2008,14 @@ TEST_CASE("Clingo propagator init", "[facade][propagator]") {
 		REQUIRE(s0.hasWatch(posLit(4), pp));
 		REQUIRE(!s0.hasWatch(posLit(3), pp));
 	}
+	SECTION("init acquires all problem vars") {
+		Var v = ctx.addVar(Var_t::Atom);
+		init.addWatch(posLit(v));
+		init.applyConfig(s0);
+		ctx.endInit();
+		PostPropagator* pp = s0.getPost(PostPropagator::priority_class_general);
+		REQUIRE(s0.hasWatch(posLit(v), pp));
+	}
 	SECTION("ignore duplicate watches from init") {
 		init.addWatch(posLit(1));
 		init.addWatch(posLit(1));