pack-objects: rework check_delta_limit usage

pack-objects: rework check_delta_limit usage

Objects that have delta "children" from pack data reuse must consider the
depth of their deepest child when they try to deltify themselves for those
children not to become too deep.

However, in the context of a "thin" pack, the delta children depth was
skipped entirely on the presumption that the pack was always going to be
exploded on the receiving end, hence the delta length wasn't an issue.

Now that we keep received packs as is and reuse pack data when repacking,
those packs do contain delta chains that are longer than expected. Worse,
those delta chain may even grow longer when the pack is further repacked
into another thin pack for a subsequent transmission.

So this patch restores strict delta length even for thin packs, and it
moves check_delta_limit() usage directly in the delta loop where it is
needed.  This way the delta_limit can be removed from struct object_entry
as well.  Oh and the initial value was wrong too.

The  progress_interval() function was moved to a more logical location in
the process.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>