t7900: prepare for switch of the default strategy

t7900: prepare for switch of the default strategy

The t7900 test suite is exercising git-maintenance(1) and is thus of
course heavily reliant on the exact maintenance strategy. This reliance
comes in two flavors:

  - One test explicitly wants to verify that git-gc(1) is run as part of
    `git maintenance run`. This test is adapted by explicitly picking the
    "gc" strategy.

  - The other tests assume a specific shape of the object database,
    which is dependent on whether or not we run auto-maintenance before
    we come to the actual subject under test. These tests are adapted by
    disabling auto-maintenance.

With these changes t7900 passes with both "gc" and "geometric" default
strategies.

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