diagnostic: refactor git origin checks and also check Cask origin

[jonchang]

• 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?

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c.f. https://discourse.brew.sh/t/issue-installing-recently-added-cask/4206/

This pull request refactors the git origin checks in brew doctor to use a common helper function, and also checks the Cask tap, if it exists, for a suspicious origin URL.

No tests written because the test framework doesn't use an actual git folder for its temporary repository.

[MikeMcQuaid]

Nice refactoring!

[MikeMcQuaid]

https://en.wikipedia.org/wiki/Rule_of_three_(computer_programming) to the rescue!

[jonchang]

Updated the wording to mirror each message. Will merge when green.

[MikeMcQuaid]

Thanks @jonchang!