R1:bd9e54b62179

Navigage Buildkite builds with more nuance

Summary:
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.

Test Plan:
- Built a commit with Buildkite.
- Build a revision with Buildkite.

Reviewers: chad

Reviewed By: chad

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam…