fetch_refs_via_pack: free extra copy of refs

fetch_refs_via_pack: free extra copy of refs

When fetch_refs_via_pack calls fetch_pack(), we pass a
list of refs to fetch, and the function returns either a
copy of that list, with the fetched items filled in, or
NULL. We check the return value to see whether the fetch was
successful, but do not otherwise look at the copy, and
simply leak it at the end of the function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>