As in the preceding commit change this API user to use strvec_pushv()
instead of assigning to the "argv" member directly. This leaves us
without test coverage of how the "argv" assignment in this API works,
but we'll be removing it in a subsequent commit.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>