audit(github): also permit refs/heads in GitHub URLs

[mahrud]

• 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 typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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This fixes the issue mentioned in #16126 (comment).

I would have added an auto-corrector, but I don't think we can choose whether the user provided a tag reference or branch reference.

[Bo98]

Thanks!