Refactor backup creation to BackupRef

Rename BackupBranch to BackupRef, and refactor backup creation out
of RefUpdateListener.

Moving the functionality into the BackupRef class will enable some
amount of configuration to how the backups are performed, and avoids
overloading too much logic in the RefUpdateListener.

Change-Id: Iced4ff7e563b6978bb29e2301a3672dc183b971e