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