t/t6501-freshen-objects.sh: prepare for `gc --cruft` by default

t/t6501-freshen-objects.sh: prepare for `gc --cruft` by default

In a similar spirit as previous commits, prepare for `gc --cruft`
becoming the default by ensuring that the tests in t6501 explicitly
cover the case of freshening loose objects not using cruft packs.

We could run this test twice, once with `--cruft` and once with
`--no-cruft`, but doing so is unnecessary, since we already test object
rescuing, freshening, and dealing with corrupt parts of the unreachable
object graph extensively via t5329.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>