This adds a new option to pack-objects which will cause it to ignore an
object which appears in a local pack which has a .keep file, even if it
was specified for packing.
This option will be used by the porcelain repack.
Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil> Signed-off-by: Junio C Hamano <gitster@pobox.com>