Update new tag-commit-push script to work around pushing branches and PRs

[mattstauffer]

Using branches & PRs means we get the benefit of CI running across all environments, not just the dev's local machine.

• [x] Make it create a branch
• [x] Drop tagging (because we can't tag until after green tests)
• [ ] Fix up the validator to actually work
• [ ] Make it create a PR using hub or whatever after pushing the PR