t: disable maintenance where we verify object database structure

t: disable maintenance where we verify object database structure

We have a couple of tests that explicitly verify the structure of the
object database. Naturally, this structure is dependent on whether or
not we run repository maintenance: if it decides to optimize the object
database the expected structure is likely to not materialize.

Explicitly disable auto-maintenance in such tests so that we are not
dependent on decisions made by our maintenance.

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