mkrep.sh now adds ReplicatingFrom: field to all generated server specs.
Previously the ReplicatingFrom: field was only added where required by
p4d, i.e. for standby type replicas. However, it's useful to have for
all server specs.
Driving reason: This is now required for HMS orchestrated global topology
upgrades, as it uses the ReplicatingFrom: field of the server spec to
determine the outer-to-inner order of upgrades.