midx docs: clarify tie breaking

midx docs: clarify tie breaking

Clarify what happens when an object exists in more than one pack, but
not in the preferred pack. "git multi-pack-index repack" relies on ties
for objects that are not in the preferred pack being resolved in favor
of the newest pack that contains a copy of the object. If ties were
resolved in favor of the oldest pack as the current documentation
suggests the multi-pack index would not reference any of the objects in
the pack created by "git multi-pack-index repack".

Helped-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>