t6500: explicitly use "gc" strategy

The test in t6500 explicitly wants to exercise git-gc(1) and is thus highly specific to the actual on-disk state of the repository and specifically of the object database. An upcoming change modifies the default maintenance strategy to be the "geometric" strategy though, which breaks a couple of assumptions. One fix would arguably be to disable auto-maintenance altogether, as we do want to explicitly verify git-gc(1) anyway. But as the whole test suite is about git-gc(1) in the first place it feels more sensible to configure the default maintenance strategy to be "gc". Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>