pack-objects: only repack or loosen objects residing in "local" packs

pack-objects: only repack or loosen objects residing in "local" packs

These two features were invented for use by repack when repack will delete
the local packs that have been made redundant.  The packs accessible
through alternates are not deleted by repack, so the objects contained in
them are still accessible after the local packs are deleted.  They do not
need to be repacked into the new pack or loosened.  For the case of
loosening they would immediately be deleted by the subsequent prune-packed
that is called by repack anyway.

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

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>