Allow jenkins admins to manage replicated git repos

We replicate all repositories from manganese to gallium, and if
a repository becomes corrupted (or renamed, or has other maintenance)
we currently have to bug a root user to handle this.

The gerritslave user only has access to these repos in /var/lib/git/*,
so there's no other files or services we could use.

Change-Id: I71837aa8cd6759a03fac3619a1e366b8fe16246c