handle_one_ref() only adds refs to the cbdata.ref_to_prune list if
(cbdata.flags & PACK_REFS_PRUNE) is set. So any references in this
list at the end of pack_refs() can be pruned unconditionally.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>