Permit redoing past an undone New Game action.

Now, when we undo a New Game by deserialising the stored version of
the previous game, we start by serialising the _current_ game into a
second serialisation buffer in the midend. Then, if the user tries to
redo back past that undo action, we can re-serialise the earlier game
and re-deserialise the newer one.

A few users had complained (with various degrees of politeness) at the
lack of this ability, because in true xkcd #1172 style, it broke their
workflow. Specifically, if you were a fan of holding down 'u' to undo
all the way back to the start of your current game, you'd have
overshot into the previous game and then have no way to return to the
game you wanted to be at the start of.

This slightly changes the previous interaction of Redo with New Game.
Previously, New Game would save the entire undo chain of the
serialised game, including anything forward of the current position;
so if you took actions move1,move2,undo,newgame then the serialised
game state would include both move1 and move2 with the current
position between them; hence, an undo would restore the old game to a
position just after move1, and then a redo action would re-enact
move2. Now, midend_purge_states is called before serialising the old
game, so in that scenario move2 would be completely lost as a side
effect of the new-game action. (Just as it would be if you made any
_other_ undoable move in that situation.)

Conversely, making a move in the old game after you've undone back
into it will now wipe out the record of the later game, so you can't
redo into it any more.

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