Fix a bug in repo perm syncing that could fetch public repos

Created by: varsanojidan

Fixed a bug where the code that fetches the repos with the stalest permissions could return a list containing public repos. This happens in a situation where a repo was once private, had permissions attached to it, and then was migrated to being public, meaning that it no longer needs permission syncing. This could theoretically cause permissions syncing to stop working if the # of repos that are migrated from private to public > the limit of repos fetched.

Test plan

Existing tests pass