Do not sort negative non captures at low depth

Speedup of the whole 3 patch series is of 2,5% !!

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/movepick.h

@@ -53,6 +53,7 @@ private:
 
   const Position& pos;
   const History& H;
+  Depth depth;
   Move ttMove;
   MoveStack killers[2];
   int captureThreshold, phase;