Cannot create PGN files in PGN collation order from databases with more than a few million games.

[RogerMarsh]

The monthly downloads from Lichess now contain up to 80 to 90 million games each.

[RogerMarsh]

The sort is attempted in one go rather than, for example, selecting games by date and sorting each date's games separately.

The actual point of failure is determined by size of available memory.

[RogerMarsh]

Commit ed86206 avoids the problem when the number of games with the same PGN Date tag value and PGN Event tag value is less than about 300,000.