fetch-pack: with packfile URIs, use index-pack arg

fetch-pack: with packfile URIs, use index-pack arg

Unify the index-pack arguments used when processing the inline pack and
when downloading packfiles referenced by URIs. This is done by teaching
get_pack() to also store the index-pack arguments whenever at least one
packfile URI is given, and then when processing the packfile URI(s),
using the stored arguments.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>