Merge changes from topic 'rc-cleanup'

* changes:
  Use refsByChange Multimap to build changeRefsById in ReceiveCommits
  Lazily load the changes byKey map in autoCloseChanges
  Simplify autoCloseCommits() handling of exact SHA-1 matches
  Cache reused values in autoCloseChanges
  Simplify parseBody call when looking at changes to close
  Refactor auto close change filtering in ReceiveCommits