Provide facility to manage multiple forks of the same upstream repository

[ddimtirov]

It is expected that in a single organization, we would have more than one individual contributors to the same upstream repository.

Example: Linda in Lisbon, Dave in Dublin, and Taro in Tokyo have all forked Node.JS and independently have worked on patches to contribute. GitProxy should recognize that these 3 upstreams have a common ancestor and allow to manage the approvals and audit of these together.

The most straight-forward implementation approach is to be able to manually link multiple proxy-repos as a group.

As a separate service, we can have repo-similarity detection service, looking for common branches, containing identical commit chains greater than a size-threshold. It is an open problem how to distinguish legit forks (i.e. OpenSSL vs. BoringSSL)