Merge branch 'master' into stable-2.11

Note that in 2.11, EventListener does not return refs/heads with all
branch names; as a result, isRelevantRef() has been modified to not
match on refs/heads or refs/tags.

* master:
  Add documentation updates
  Add details from original tag
  Add ability to back up as tag
  Correct backup of tags
  Switch to EventListener
  Add configuration for backup branch names
  Add configuration for levels of protection
  Fix MissingObjectException when checking for non-ff update
  Refactor backup creation to BackupRef

Change-Id: I9df9a60870c433730c6ae2f946688c8d902e0221