objects to be pruned immediately don't have to be loosened

objects to be pruned immediately don't have to be loosened

When there is no grace period before pruning unreferenced objects, it is
pointless to push those objects in their loose form just to delete them
right away.

Also be more explicit about the possibility of using "now" in the
gc.pruneexpire config variable (needed for the above behavior to
happen).

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>