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

Maniphest Tasks: T12173

Differential Revision: https://secure.phabricator.com/D17282