perf: disable automatic housekeeping

Turn off automatic background maintenance for perf tests by default to
avoid interference with performance measurements. Do that by using the
new file t/perf/config and using it as the system config file for perf
tests. Future tests intended to measure gc performance can override
the setting locally or call "git gc" explicitly.

This fixes a breakage in p2000 caused by gc automatically emptying the
reflog due its fake dates from 2005 being older than 90 days.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>