gc.sh: sleep before touching primary if any supplemental

gc.sh: sleep before touching primary if any supplemental

Git sorts the packs it uses by modification time.  It looks in the
most recently modified packs first.

We want it to look in our nice new shiny .bitmap pack first after
a fresh gc has been performed.

We touch the pack to make sure this happens.

However, it's possible that if we just finished building a supplemental
pack and didn't have anything to do for any forks that it could end
up with the same modification time (to the nearest second) as the
supplemental pack ended up with.

Avoid this by sleeping for a second before touching if any supplemental
pack was created during phase II.

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