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

diff --cc cache.h
Simple merge