download_strategy: support Mercurial tags/branches

[EricFromCanada]

• 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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The pypy3.9 and pypy3.10 formulae each have a branch set for their Mercurial-based head URLs, but it turns out those have been ignored until now. This adds support for tag:, revision:, and branch: for Mercurial repository URLs, with which you can:

1. add e.g. branch: "dev" to a head URL after using: :hg and it will work, e.g.: brew fetch --HEAD pypy3.10; hg branch --cwd $(brew --cache --HEAD pypy3.10) should print "py3.10" instead of "default"
2. set a formula to fetch a release directly from a Mercurial tag, e.g.:

  url "https://hg.sr.ht/~breakfastquay/rubberband", using: :hg,
      tag:      "v3.3.0",
      revision: "55367f2e76c9"

[MikeMcQuaid]

Makes sense, thanks again @EricFromCanada!