Add check for HEAD ref in diagnostics

[BenMusch]

• 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 tests with your changes locally?

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Addresses this issue: #3068 by extending a diagnostic check so that it adds a warning to the console if .git/HEAD is not set to /refs/heads/master

Any thoughts on how to test this? Are there patterns established for testing things that interact with the filesystem?

[MikeMcQuaid]

Nice work! Just the messaging and a small code tweak and this should be good to go.

[MikeMcQuaid]

Don't use unless ... &&; it's hard to follow. Switch to an if.

[MikeMcQuaid]

Replace all the above text with something like:

Homebrew/homebrew-core is not on the master branch

Check out the master branch by running:
  git -C "$(brew --repo homebrew/core)" checkout master

Want to avoid Git jargon where possible here.

[MikeMcQuaid]

Fantastic work again, @BenMusch! Looking forward already to your next PR 😍

[MikeMcQuaid]

@BenMusch this had to be reverted unfortunately (see the linked stuff above) but I’d love if you could try again in a new PR and detail what commands you use to test this so we can try them too. Thanks again!

[BenMusch]

Interesting, will investigate and re-open. Thanks for the heads up @MikeMcQuaid

[ilovezfs]

Thanks, @BenMusch! For what it's worth, I think you need to use --symbolic-full-name.