R1:49b718178066

Update utility "bin/repository parents" workflow to work with RefPosition

Summary:
Ref T11823. I think this is the last callsite which relies on the old data format: `bin/repository parents` rebuilds a cache which we don't currently use very heavily.

Update it to work with the new data.

Test Plan: Ran `bin/repository parents <repository> --trace`, saw successful script execution and reasonable-looking output.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11823

Differential Revision…