bump-formula-pr: restore formula if duplicate PR exists

[SeekingMeaning]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

I figured this made sense since formulae are restored for bumps that are downgrades or don't actually change the version: https://github.com/Homebrew/brew/blob/master/Library/Homebrew/dev-cmd/bump-formula-pr.rb#L332-L344

[maxim-belkin]

I wonder if bump-formula-pr should even do something if there is an open PR already? Shall it, perhaps, do this check early and bail out if there is an open pull request?

[SeekingMeaning]

Yes, we can do the open pull requests check early, although the closed PRs check will have to stay since it relies on knowing what's the latest version

Not sure if I should make this change in this PR or another

[jonchang]

It's fine to make it in a new pull request. This is handy, thanks!

[maxim-belkin]

backup_file is a slightly misleading variable name -- it's actually old_contents rather than backup_file (which we could mv instead of having to .atomic_write it). Changing the name is easy so maybe @SeekingMeaning could do that in a new PR (I don't want to get ownership of lines just for changing the variable name).

Also, we need to make sure backup_file (or old_contents) is defined for dry-runs as well.