cmd/atlas/internal/cmdapi: new "schema diff" flag to error out on diff.

[pylaligand]

This makes it easier to programmatically verify that migrations are up-to-date with e.g. an ORM-generated schema in a CI context by observing the command's return code rather than parsing its output.

[pylaligand]

Happy new year! Anyone to review this? @a8m maybe?

[a8m]

Thanks for the contribution, @pylaligand. Currently, in order to implement something like this in CI, you can do it this way:

atlas schema diff \
  --from "file://migrations" \
  --to env://orm \
  --env local \
  --format '{{ if .Changes }}The failure message{{ end }}' | grep '.' && exit 1

Or, convert it to if [ -n "$output" ]; then .. fi.

This is flexible enough because users can control the error message and the format of the failure. If we'll add such flag, we also need to support a way to customize its message, which I prefer to avoid.

[pylaligand]

Makes sense, thanks.