Do not claim repetition after null move

Null moves can artificially create a repetition
draw where instead there is no one.

So use a second counter to reset history after
a null move.

Idea from Joona.

After 999 games at 1+0

Mod vs Orig +238 =553 -208 51.50%  514.5/999  +10 ELO

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

src/position.h

@@ -88,7 +88,7 @@ enum Phase {
 
 struct StateInfo {
   Key key, pawnKey, materialKey;
-  int castleRights, rule50;
+  int castleRights, rule50, pliesFromNull;
   Square epSquare;
   Value mgValue, egValue;
   Value npMaterial[2];