delta-islands: respect progress flag

delta-islands: respect progress flag

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: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>