Support for Gitaly opt-ins in Gitlab 10.5
This is an important step towards supporting GitLab versions from 11 onwards, where Gitaly is essentially mandatory.
This is also really useful for development with the GitLab Development Kit (GDK), in which all Gitaly opt-ins are activated by default. In previous experiments with the GDK, we had to hardcode a blanket disabling of all Gitaly features.
Not to be confused with the HGitaly project, what we are doing here is let GitLab go through Gitaly for the inner auxiliary Git repos.