run-command API users: use strvec_push(), not argv construction

run-command API users: use strvec_push(), not argv construction

Change a pattern of hardcoding an "argv" array size, populating it and
assigning to the "argv" member of "struct child_process" to instead
use "strvec_push()" to add data to the "args" member.

As noted in the preceding commit this moves us further towards being
able to remove the "argv" member in a subsequent commit

These callers could have used strvec_pushl(), but moving to
strvec_push() makes the diff easier to read, and keeps the arguments
aligned as before.

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