midx: use strvec_pushf() for pack-objects base name

midx: use strvec_pushf() for pack-objects base name

Build the pack base name argument directly using strvec_pushf() instead
of with an intermediate strbuf.  This is shorter, simpler and avoids the
need for explicit cleanup.

Signed-off-by: René Scharfe <l.s.r@web.de>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>