The commit-graph file has an extra chunk to store the parent int-ids for
parents beyond the first parent for octopus merges. Our test repo has a
single octopus merge that we can manipulate to demonstrate the 'verify'
subcommand detects incorrect values in that chunk.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>