t6500: explicitly use "gc" strategy

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>