This check is a precondition for a branch to be deletable regardless
of permissions. PermissionBackend should only check whether the
user is granted permission to do an action.
For the "refs/meta/config" branch, we will not allow anyone to delete
this branch even though somehow this user has been granted the DELETE
permission.
After this commit, every time we check the DELETE permission, we
need to verify it's not the "refs/meta/config" branch which is a
burden to every caller.
Change-Id: Ic60302d5dc567d92f52b6a237b368f3f8ff7445c
5087440fda