Runner ability to fetch refs for merge request pipelines

gitlab-org/gitlab-ee#7380 will introduce building combined refs for merge request pipelines.

GitLab will need to schedule jobs for refs/merge-requests/:iid:/merge ref that is outside of the default cloning ref-spec.

Today users can control how they retrieve source code (clone, fetch, or none). Given there's no way to clone outside of the default ref-spec, we will force a fetch mode in case of merge request pipeline.