gc.sh: trigger mini gc in forks if needed on copy down

gc.sh: trigger mini gc in forks if needed on copy down

When the packs from a parent are copied (actually hard linked) down into
any forks before beginning gc on the parent, this could bump the number
of packs in a fork above the threshold for a mini gc.

Detect this and set .needsgc in this case.

Although this is unlikely in practice since the parent does attempt to
combine small packs in this case before copying (linking) the packs down
to any forks, it could still happen if the parent has many large packs
or the fork was on the verge of triggering already so it's good to do
the check since it's very cheap.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>