builtin/maintenance: fix loose objects task emitting pack hash

builtin/maintenance: fix loose objects task emitting pack hash

The "loose-objects" maintenance tasks executes git-pack-objects(1) to
pack all loose objects into a new packfile. This command ends up
printing the hash of the packfile to stdout though, which clutters the
output of `git maintenance run`.

Fix this issue by disabling stdout of the child process.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>