Add --ref option to start command

Users can now trigger replication for a specific reference using the
--ref <RefName> option. This allows for targeted replication of
individual branches or tags rather than forcing a full project sync.

Example usage:
  ssh -p 29418 host replication start --ref refs/heads/master my-project

Change-Id: If6d208411e2a1645fff4f7a6d2e6c5428b87e3f3
Release-Notes: Add --ref <RefName> option to start command