Ref T12173.

• If we want to fetch a tag, Buildkite needs it as a "branch" (this means more like "ref to fetch").
• The API gets upset if we pass "refs/tags/...", so just pass the tag name without the prefix, which works.
• Do a better job with commits and pass a real branch to fetch.

• Built a commit with Buildkite.
• Build a revision with Buildkite.