Reviewly

Reviewly helps companies and tech recruiters to manage the take-home projects that they send to job candidates. It allows them to create take-home projects where they can ask for coding and also discuss an issue and/or review a pull request directly on GitHub.

It allows to prepare the main repository once, and then takes care of replicating everything in a new private repository for each candidate.
It's also possible to integrate existing tools with Reviewly through Zapier.