fsck: check rev-index position values

fsck: check rev-index position values

When checking a rev-index file, it may be helpful to identify exactly
which positions are incorrect. Compare the rev-index to a
freshly-computed in-memory rev-index and report the comparison failures.

This additional check (on top of the checksum validation) can help find
files that were corrupt by a single bit flip on-disk or perhaps were
written incorrectly due to a bug in Git.

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