pack-objects: show some progress when counting kept objects

pack-objects: show some progress when counting kept objects

We only show progress when there are new objects to be packed. But
when --keep-pack is specified on the base pack, we will exclude most
of objects. This makes 'pack-objects' stay silent for a long time
while the counting phase is going.

Let's show some progress whenever we visit an object instead. The old
"Counting objects" is renamed to "Enumerating objects" and a new
progress "Counting objects" line is added.

This new "Counting objects" line should progress pretty quick when the
system is beefy. But when the system is under pressure, the reading
object header done in this phase could be slow and showing progress is
an improvement over staying silent in the current code.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>