make pack-objects a bit more resilient to repo corruption

make pack-objects a bit more resilient to repo corruption

Right now, packing valid objects could fail when creating a thin pack
simply because a pack edge object used as a preferred base is corrupted.
Since preferred base objects are not strictly needed to produce a valid
pack, let's not consider the inability to read them as a fatal error.
Delta compression may well be attempted against other objects in the
search window.  To avoid warning storms (we are in the inner loop of
the delta search window) a warning is emitted only on the first
occurrence.

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