Don't print set git credential cache message if a helper exists

[xxyzz]

when a private repository is tapped

• 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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Return earlier if git credential helper is already used. Resolves #13097.

[MikeMcQuaid]

Thanks so much for your contribution! Without people like you submitting PRs we couldn't run this project. You rock, @xxyzz!