gitlab-ci: Add tests for reftable backend

The reftable backend is soon to be pushed upstream and might be released in the next few git versions. Since the release is around the horizon, it becomes ever so important that we test our repo against the new backend to catch and fix bugs early in the lifecycle.

To support this, we add a new job to the GitLab CI to test against the reftable backend. Since the reftable backend isn't officially merged to next/master branches of git, we rely on the pks-reftables-initial-baseline integration branch.

We also expose and use a new env variable GITALY_TEST_WITH_REFTABLE, which when set, appends the --ref-format=reftable option to git-init(1) and git-clone(1).

Eventually we can get rid of the integration branch when the patch is accepted upstream.

Closes #5766 (closed)