threefold/fivefold repetition checks should consider only current state

Way back in 4cabc410, I thought it was important to allow users to claim a draw at any point in the future after a threefold repetition. But this is not what the laws of chess allow: it should be claimed when it happens, not at random points in the future. At the time, that change probably made sense because we didn't tell the user when it's possible to claim a draw, which nowadays we do by opening a message dialog. But nowadays, it has no benefit, and a large cost: the message dialog appears at the start of every turn, forevermore, getting in the way and irritating the user. So let's only show it if the current board state really is eligible for a draw.

Fixes #47 (closed)