Undo size improvements

[curiousdannii]

First, the default should probably be bumped from 256 to 1024, if not even higher.

Second, allow the size to be specified at runtime as Git does.

[curiousdannii]

I've increased it to 4096 (in the emglken branch) which seems to allow you to undo several turns in Cryptozookeeper, at the beginning of the game at least. We may want an even higher default size - advice would be welcome!