Check for 3-fold repetition on the first move of PV

Other moves are tested for single repetition as usual.
This compromise intends to fix inaccurate draw scores
on first repetition when analyzing games played by humans.
Since the same pruning occurs during the search thanks
to 2-fold repetition check, the engine shouldn't lose any
strength.
Improve readability
Merge branch 'master' into 3fold_1stMove
Merge branch 'master' into 3fold_1stMove
Merge branch '3fold_1stMove' of https://github.com/promaret/Stockfish into tactical