Reproducible searches after ucinewgame

Fixes issue #859.

thisThread->callsCnt in search<>() was different (by 1) for the first and second game played.

No functional change.

src/search.cpp

@@ -212,6 +212,7 @@ void Search::clear() {
       th->counterMoves.clear();
       th->fromTo.clear();
       th->counterMoveHistory.clear();
+      th->resetCalls = true;
   }
 
   Threads.main()->previousScore = VALUE_INFINITE;