Merge branch 'ref-abstract'

* ref-abstract:
  Optimize RefAdvertiser performance by avoiding sorting
  branch: Add -m option to rename a branch
  Replace writeSymref with RefUpdate.link
  Rewrite reference handling to be abstract and accurate
  Create new RefList and RefMap utility types

Change-Id: If43aacf5aa4013edbd0a6e84d84c4f9e94de5be0