Merge branch 'master' into stable-7.2

* master:
  Ensure access to autoRefresh is thread-safe
  FileReftableStack: use FileSnapshot to detect modification
  FileReftableDatabase: consider ref updates by another process
  BlameRegionMerger: report invalid regions with checked exception.
  [ssh known_hosts] Handle unknown keys better
  [releng] Remove unused target platform definitions

Change-Id: I2e49d582a5be9f1383b4f1713e33cb378b4f6118