git: Sync internal namespaces with distros' settings

Repositories hosted by Gitaly have several internal reference namespaces that shouldn't be modified by clients e.g. by pushing to them. And while we have the infrastructure in place already to ensure that writes are restricted, the list of internal namespace is not in sync with the list of namespaces configured in both Omnibus and CNG. Most notably, we're missing both the refs/tmp/ and refs/remotes/ namespaces.

Add the missing namespaces to ensure that we're not regressing our safety guards in this area when we remove the gitconfig in our distros.

Changelog: changed

Closes #4237 (closed)