pack-objects: refactor code into compute_layer_order()

pack-objects: refactor code into compute_layer_order()

In a following commit, as we will use delta islands, we will
have to compute the write order for different layers, not just
for one.

Let's prepare for that by refactoring the code that will be
used to compute the write order for a given layer into a new
compute_layer_order() function.

This will make it easier to see and understand what the
following changes are doing.

Helped-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>