Join refutation stages in the movepicker

Unifies a bit further the three refuation stages in the MovePicker
class. Also treat the skipping of TT move now always via select_move(),
as discussed in pull request #1454.

Passed STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 16608 W: 3461 L: 3331 D: 9816
http://tests.stockfishchess.org/tests/view/5ab0aaf00ebc59029fb6f6c3

Closes https://github.com/official-stockfish/Stockfish/pull/1502

No functional change.

src/movepick.h

@@ -131,8 +131,8 @@ private:
   const ButterflyHistory* mainHistory;
   const CapturePieceToHistory* captureHistory;
   const PieceToHistory** contHistory;
-  Move ttMove, refutations[3];
-  ExtMove *cur, *endMoves, *endBadCaptures;
+  Move ttMove;
+  ExtMove refutations[3], *cur, *endMoves, *endBadCaptures;
   int stage;
   Move move;
   Square recaptureSquare;