R1:4c2696120b81

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…