pack-objects: allow "fixing" a corrupted pack without a full repack

pack-objects: allow "fixing" a corrupted pack without a full repack

When the pack data to be reused is found to be bad, let's fall back to
full object access through the generic path which has its own strategies
to find alternate object sources in that case.  This allows for "fixing"
a corrupted pack simply by copying either another pack containing the
object(s) found to be bad, or the loose object itself, into the object
store and launch a repack without the need for -f.

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