R1:2a203fbab1ff

Add proper PHIDs to RefCursors

Summary: Ref T9952. See discussion there. This change is primarily aimed at letting me build a typeahead of branches in a repository so that we can land to arbitrary branches a few diffs from now.

Test Plan:
- Ran migrations.
- Verified database populated properly with PHIDs (`SELECT * FROM repository_refcursor;`).
- Ran `bin/repository update`.
- Viewed a Git repository in Diffusion.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9952

Differential Revision…