config: let feature.experimental imply gc.cruftPacks=true

config: let feature.experimental imply gc.cruftPacks=true

We are interested in exploring whether gc.cruftPacks=true should become
the default value.

To determine whether it is safe to do so, let's encourage more users to
try it out.

Users who have set feature.experimental=true have already volunteered to
try new and possibly-breaking config changes, so let's try this new
default with that set of users.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>