let pack-objects do the writing of unreachable objects as loose objects

let pack-objects do the writing of unreachable objects as loose objects

Commit ccc1297226b184c40459e9d373cc9eebfb7bd898 changed the behavior
of 'git repack -A' so unreachable objects are stored as loose objects.
However it did so in a naive and inn efficient way by making packs
about to be deleted inaccessible and feeding their content through
'git unpack-objects'.  While this works, there are major flaws with
this approach:

- It is unacceptably sloooooooooooooow.

  In the Linux kernel repository with no actual unreachable objects,
  doing 'git repack -A -d' before:

	real    2m33.220s
	user    2m21.675s
	sys     0m3.510s

  And with this change:

	real    0m36.849s
	user    0m24.365s
	sys     0m1.950s

  For reference, here's the timing for 'git repack -a -d':

	real    0m35.816s
	user    0m22.571s
	sys     0m2.011s

  This is explained by the fact that 'git unpack-objects' was used to
  unpack _every_ objects even if (almost) 100% of them were thrown away.

- There is a black out period.

  Between the removal of the .idx file for the redundant pack and the
  completion of its unpacking, the unreachable objects become completely
  unaccessible.  This is not a big issue as we're talking about unreachable
  objects, but some consistency is always good.

- There is no way to easily set a sensible mtime for the newly created
  unreachable loose objects.

So, while having a command called "pack-objects" to perform object
unpacking looks really odd, this is probably the best compromize to be
able to solve the above issues in an efficient way.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>