Add --exit-code flag to spoom bump

[JamieMagee]

Adding an --exit-code flag to spoom bump, similar to git diff, which sets an exit code if there are files that can be bumped will make spoom bump easier to block builds when someone forgot to bump the strictness of a file.

Currently, the workaround I am using a bash script like:

if bundle exec spoom bump --from=false --to=true | grep 'Bumped'; then
    echo "Files changed by spoom bump. Please run \`bundle exec spoom bump --from=false --to=true\` locally and commit the changes."
    exit 1
fi

See dependabot/dependabot-core#8469 for more information.

[Morriar]

Isn't it already the case? You can also use --dry to avoid actually bumping the files.

You can see how we use it on this repo CI step: https://github.com/Shopify/spoom/blob/main/.github/workflows/ruby.yml#L30-L31.

[JamieMagee]

Thanks! The documentation doesn't note this, and I didn't dig into the source code.

[Morriar]

#490