t7900: clean up large EXPENSIVE repository

t7900: clean up large EXPENSIVE repository

One of the tests in t7900 is marked with EXPENSIVE because we create a
repository with 2GB of data that we end up repacking. We never clean up
that repository though, so we occupy the full 2GB of data until the end
of the test suite.

Besides clogging our disk, having an EXPENSIVE test that alters the
repository's state used by subsequent tests is also a bad idea, as it
can easily have an impact on the heuristics used by other maintenance
tasks.

Adapt the test so that we create the data in a standalone repository
that we clean up at the end of the test. While at it, also disable
auto-maintenance so that it does not race with our manual maintenance.

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