fetch-pack: support protocol version 2

fetch-pack: support protocol version 2

When the scaffolding for protocol version 2 was initially added in
8f6982b4e1 ("protocol: introduce enum protocol_version value
protocol_v2", 2018-03-14). As seen in:

    git log -p -G'support for protocol v2 not implemented yet' --full-diff --reverse v2.17.0..v2.20.0

Many of those scaffolding "die" placeholders were removed, but we
hadn't gotten around to fetch-pack yet.

The test here for "fetch refs from cmdline" is very minimal. There's
much better coverage when running the entire test suite under the WIP
GIT_TEST_PROTOCOL_VERSION=2 mode[1], we should ideally have better
coverage without needing to invoke a special test mode.

1. https://public-inbox.org/git/20181213155817.27666-1-avarab@gmail.com/

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>