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: https://secure.phabricator.com/D14731