Ensure that head Git URLs always specify a branch name

[issyl0]

• 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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• There's a TODO on the "someday" list 1 to ensure that head Git URLs always specify a branch. So I thought I'd automate this worry by adding an audit.
• Since resource block URLs tend to be pinned to SHAs, if indeed they are Git URLs, this audit only applies to head URLs.
• Since head must now specify a url and branch, the head do block with only these stanzas can be condensed to the single-line head "url", branch: "branch" format.
• Always specify a branch if we can find one. If a HEAD branch name isn't specified at all, then the user probably wants to shortcut adding one by being told what the default branch for the repo is. Otherwise they have to click the URL, look at the GitHub UI, then type the branch name into branch: "foo" syntax.

[issyl0]

Hmmm. Homebrew/homebrew-test-bot#1540 is merged and the workflow run here says it's Using Homebrew/homebrew-test-bot 39e056b (Merge pull request #1540 from Homebrew/testbottest-head-branch). And yet:

Full audit --formula homebrew/test-bot/testbottest --online --git --skip-style output
  homebrew/test-bot/testbottest
    * HEAD: Git `head` URL must specify a branch name