`try-summary` could generate commit shell script

The `try-summary` utility of #158 generates a human-readable summary of the changes in the upperdir. We currently use separate commit logic, but we could additionally/instead generate a shell script while computing the summary.

As a pro, this shell script would be an inspectable, durable artifact. It would also save a traversal during commit logic.

As a con, this shell script might end up being slower than `try-commit`, which only sometimes has invoke `rm` or `mv`.

On balance, this feature is currently unplanned. If implementing this behavior would help you, please leave a comment here to let us know.