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: https://secure.phabricator.com/D18615