The each_ref_fn add_existing() adds refnames to the existing_refs
list. But the lifetimes of these refnames is not guaranteed by the
refs API, so configure the string_list to make copies as it adds them.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>