Remove DiffusionBranchInformation in favor of DiffusionRepositoryRef

Summary: Ref T4327. At some point these two very similar classes got introduced. Collapse DiffusionBranchInformation into the nearly identical DiffusionRepositoryRef, which enjoys slightly more generality and support.

Test Plan: Viewed branch overview and detail pages. Ran repository refs and repository discover. Grepped for removed symbols.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4327

Differential Revision: https://secure.phabricator.com/D8002