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

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• Have you written new tests for your changes? Here's an example.
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[benknoble]

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