Precompute repetition info (#2132)

Store repetition info in StateInfo instead of recomputing it in
three different places. This saves some work in has_game_cycle()
where this info is needed for positions before the root.

Tested for non-regression at STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 34104 W: 7586 L: 7489 D: 19029
http://tests.stockfishchess.org/tests/view/5cd0676e0ebc5925cf044b56

No functional change.

src/position.h

@@ -46,6 +46,7 @@ struct StateInfo {
   Square epSquare;
 
   // Not copied when making a move (will be recomputed anyhow)
+  int repetition;
   Key        key;
   Bitboard   checkersBB;
   Piece      capturedPiece;