Allow force running of --once if current.sql has not changed

### Feature description

Often we use tricks like `select n` and increment the number or change `current.sql` in another way to get it to run.

It would be handy to have a flag to force run `--once` if the file hasn't changed.

### Motivating example

When testing a data migration, it is often very handy to have an idempotent migration that can get run multiple times as you add/change data in your application.

Typical workflow for me in a data migration involves
- Keep the original data I am moving in place (if possible)
- Run the migration, check for base functionality, compare against existing
- Add more data to test the migration and run it again (rinse and repeat as needed)
- Once tested, add code to cleanup the original data, test again, and ship

Though workarounds are simple as described in the feature description, I think it would be a nice add.

### Breaking changes

n/a

### Supporting development

I [tick all that apply]:

- [ ] am interested in building this feature myself
- [ ] am interested in collaborating on building this feature
- [ x ] am willing to help testing this feature before it's released
- [ ] am willing to write a test-driven test suite for this feature (before it exists)
- [ x ] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ x ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile

Start by tracing the --once handling and the logic that detects whether current.sql has changed. Determine how a force option should interact with that check, then verify the behavior with the existing migration tests or test entry points you find; done means an unchanged current.sql can be run repeatedly when explicitly forced.

Written by the indexing model from the issue text.