fsck: use ERROR_MULTI_PACK_INDEX
The multi-pack-index was added to the data verified by git-fsck in
ea5ae6c3 "fsck: verify multi-pack-index". This implementation was
based on the implementation for verifying the commit-graph, and a
copy-paste error kept the ERROR_COMMIT_GRAPH flag as the bit set
when an error appears in the multi-pack-index.
Add a new flag, ERROR_MULTI_PACK_INDEX, and use that instead.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>