HBASE-19165 is about starting a branch-2 over branch-1 dataset. The Master when it comes up, goes to read the 'state' of the region from hbase:meta because this is how region state is kept now by AMv2 only, there is no state in hbase:meta on a fresh start. In hbase1, region state was kept in master memory persisted to zk (rather than hbase:meta) while the region was in transition.
We need to add to master a self-migration facility. If a region has no state in hbase:meta, then look in zk to see if it RIT. If nothing in RIT, ask the pertinent regionserver if it has the region.