Add GitHub mirror to list of known mirrors

Summary: Ref T15046

Test Plan: Clone phorge from github mirror, visit /config/, see correct branch-point in Version Information

Reviewers: O1 Blessed Committers, Cigaryno, valerio.bozzolan

Reviewed By: O1 Blessed Committers, Cigaryno, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15046

Differential Revision: https://we.phorge.it/D25115