midx: mark bad packed objects

midx: mark bad packed objects

When an object fails to decompress from a pack-file, we mark the object
as 'bad' so we can retry with a different copy of the object (if such a
copy exists).

Before now, the multi-pack-index did not update the bad objects list for
the pack-files it contains, and we did not check the bad objects list
when reading an object. Now, do both.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>