feat(tap-info): display Git information about non-Core/non-API taps

[benknoble]

Copy and tweak code from Library/Homebrew/system_config.rb:110 to commit and date information. This information can be useful when debugging formulae in custom taps to ensure the tap has been correctly updated recently or to suss out important differences from one version of a tap to another.

This removes the need to ask users to run something like

git -C $(brew --repository)/Library/Taps/<tap> show -s --decorate

Close #18381

• Have you followed the guidelines in our Contributing document?
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• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
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• Have you successfully run brew tests with your changes locally?

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[benknoble]

I have some more work to properly prepare the PR (see checklist)

[MikeMcQuaid]

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