RefService: implement DeleteRefs
Delete special refs, keep treating deletion attempts for branches and tags as hard errors.
Also part of heptapod#431 (closed) and needed to conclude %HGitaly1: Hg SHAs for the same reasons as #48 (closed)
Delete special refs, keep treating deletion attempts for branches and tags as hard errors.
Also part of heptapod#431 (closed) and needed to conclude %HGitaly1: Hg SHAs for the same reasons as #48 (closed)