Tweak best move history updates.
If not at an PV node, scale history bonus for the found best move up proportionally to the count of the other moves searched.
Passed STC:
LLR: 2.96 (-2.94,2.94) <0.00,2.00>
Total: 219712 W: 56104 L: 55525 D: 108083
Ptnml(0-2): 565, 25482, 57188, 26051, 570
https://tests.stockfishchess.org/tests/view/6a1306fb818cacc1db0ac5b7
Passed LTC:
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 53988 W: 13851 L: 13498 D: 26639
Ptnml(0-2): 22, 5779, 15051, 6108, 34
https://tests.stockfishchess.org/tests/view/6a1b557e818cacc1db0ad0b8
closes https://github.com/official-stockfish/Stockfish/pull/6871
Bench: 2935474