pack-objects: move tree_depth into 'struct packing_data'

pack-objects: move tree_depth into 'struct packing_data'

This reduces the size of 'struct object_entry' and therefore
makes packing objects more efficient.

This also renames cmp_tree_depth() into tree_depth_compare(),
as it is more modern to have the name of the compare functions
end with "compare".

Helped-by: Jeff King <redacted>
Helped-by: Duy Nguyen <redacted>
Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>