Now _this_ is what Undo ought to be doing in a Minesweeper clone.
Rather than revealing the entire mine layout when you die, we now
only reveal the one mine that killed you. You can then Undo and
continue playing, without having spoiled the rest of the grid for
yourself. The number of times you've died is counted in the status
line (and is not reduced by Undo :-).

Amusingly, I think this in itself is quite a good way of dealing
with ambiguous sections in a Minesweeper grid: they no longer
_completely_ spoil your enjoyment of the game, because you can still
play the remainder of the grid even if you haven't got a completely
clean sweep. Just my luck that I should invent the idea when I've
already arranged for ambiguous sections to be absent :-)

[originally from svn r5886]