As a side effect of the Diffusion->GitLab migration we have Diffusion mirrors for all of the repos that we migrated to GitLab. There is some utility in having a Diffusion mirror, most directly the ability to link to particular commits with a git hash in a Phabricator task.
Repurpose the existing Diffusion repo management code in Striker create a mirror at the same time as creating a new GitLab repo.