git undo enhancements

[arxanas]

• Add non-interactive mode as the default. This would just undo the last operation, which is the most common case. It's also a lot simpler to demo.
• Include the git command that was being run as part of the transaction.
• Include some UI controls to handle going back and forth, and don't just rely on the keyboard.
• Checkout operations don't use the standard progress reporting mechanism, so it might appear that git undo is hanging when it's doing a long checkout operation.