Ensure that `head` Git URLs always specify a branch

- 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.

[1]: https://github.com/orgs/Homebrew/projects/5?pane=issue&itemId=98789749