Transfer ownership of detached strbufs to string_lists of the
duplicating variety by calling string_list_append_nodup() instead of
string_list_append() to avoid duplicating and then leaking the buffer.
While at it make sure to release the string_list when done;
push_refs_with_export() already does that.
Reported-by: Jeff King <peff@peff.net> Signed-off-by: Rene Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>