bump-formula-pr: make --dry-run conflict with --write

[SeekingMeaning]

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Related: #8177 (comment)

I'd expect --dry-run and --write to conflict and there to be a e.g. --no-commit if you want to avoid committing (this is also what brew bottle does with --write and --no-commit).

This PR keeps the existing behavior for --write, i.e. it does not perform any git operations by default. To create a commit without opening a pull request, we can use the new --commit switch

[MikeMcQuaid]

Makes sense to me, nice work 👍🏻