pack-objects: don't loosen objects available in alternate or kept packs

pack-objects: don't loosen objects available in alternate or kept packs

If pack-objects is called with the --unpack-unreachable option then it
will unpack (i.e. loosen) all unreferenced objects from local not-kept
packs, including those that also exist in packs residing in an alternate
object database or a locally kept pack.  The only user of this option is
git-repack.

In this case, repack will follow the call to pack-objects with a call to
prune-packed, which will delete these newly loosened objects, making the
act of loosening a waste of time.  The unnecessary loosening can be
avoided by checking whether an object exists in a non-local pack or a
locally kept pack before loosening it.

This fixes the 'local packed unreachable obs that exist in alternate ODB
are not loosened' test in t7700.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>