commit-graph: test octopus merges with --split

commit-graph: test octopus merges with --split

Octopus merges require an extra chunk of data in the commit-graph
file format. Create a test that ensures the new --split option
continues to work with an octopus merge. Specifically, ensure
that the octopus merge has parents across layers to truly check
that our graph position logic holds up correctly.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>