Merge branch 'jh/verify-index-checksum-only-in-fsck'

The index file has a trailing SHA-1 checksum to detect file corruption, and historically we checked it every time the index file is used. Omit the validation during normal use, and instead verify only in "git fsck". * jh/verify-index-checksum-only-in-fsck: read-cache: force_verify_index_checksum