bundle API: change "flags" to be "extra_index_pack_args"

bundle API: change "flags" to be "extra_index_pack_args"

Since the "flags" parameter was added in be042aff24c (Teach progress
eye-candy to fetch_refs_from_bundle(), 2011-09-18) there's never been
more than the one flag: BUNDLE_VERBOSE.

Let's have the only caller who cares about that pass "-v" itself
instead through new "extra_index_pack_args" parameter. The flexibility
of being able to pass arbitrary arguments to "unbundle" will be used
in a subsequent commit.

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