Summary: Ref T3656. Releeph denormalizes branch cut point identifiers into Branch objects, but this information isn't useful or used for sorting, filtering, or enforcing unique constraints. Instead, derive it via noramlized pathways from the `cutPointCommitPHID`.
Test Plan: Ran storage upgrade. Ran `releephwork.getbranch` and `releeph.getbranches`. Grepped for `cutPointCommitIdentifier`.
Reviewers: btrahan
Reviewed By: btrahan
CC: LegNeato, aran
Maniphest Tasks: T3656
Differential Revision: https://secure.phabricator.com/D6636
