Merge branch 'stable-3.1' into stable-3.2

* stable-3.1 (27 commits)
  Use shallow clone while expanding definitions
  Move match() and matchOrNull() to MatchCache
  fixup! Cache Task definition lists for ChangeNodes
  Cache Task definition lists for ChangeNodes
  Split SubNodeFactory out of SubNodeAdder
  Moving caching logic out of loadSubNodes()
  Use a TaskTree.Node.Invalid instead of nulls
  Use overloading for special cases instead of nulls
  Add a TaskTree ApplicableNodeFilter
  Add an isCacheableByBranch() to the PredicateCache
  Track whether Task.applicable needs to be refreshed
  Allow Expander to expand a single Task's field
  Minor cleanup of task Properties.expandText()
  Remove cached Change subNodes on Node completion
  Explicitly signal end of Task Properties expansion
  Use a SubNodeAdder to add TaskTree SubNodes
  Avoid hard coding the refs/meta/config ref
  Inject All-Projects in TaskConfigFactory
  Change TaskExpressions to iterate over TaskKeys
  Avoid passing isTrusted in TaskConfigFactory.getTaskConfig()

Change-Id: I783f00289dff687f066d395862ec790e2fe058b5