Merge branch 'stable-2.8'

* stable-2.8:
  PatchListCacheImpl: explicitly check for null revision
  ChangeField: skip PatchSets with null revisions
  Rename methods in PluginConfigFactory to be more explicit
  Explain limitation of storing plugin config in existing config files

Change-Id: Id34a619c8979e2d3048244a9fb3d2c03482ca952