The delta island code always prints "Marked %d islands", even if
progress has been suppressed with --no-progress or by sending stderr to
a non-tty.

Let's pass a progress boolean to load_delta_islands(). We already do
the same thing for the progress meter in resolve_tree_islands().
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>