builtin/repack.c: move `.idx` files into place last

builtin/repack.c: move `.idx` files into place last

In a similar spirit as the previous patch, fix the identical problem
from `git repack` (which invokes `pack-objects` with a temporary
location for output, and then moves the files into their final locations
itself).

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