Fix checkout_opts[merges] for lhcb-master-mr slots
Special merges instructions (e.g. label=lhcb-master) were overwritten in the test slots (e.g. lhcb-master-mr), which is a bug.
Special merges instructions (e.g. label=lhcb-master) were overwritten in the test slots (e.g. lhcb-master-mr), which is a bug.