commit-graph: test verify across alternates

commit-graph: test verify across alternates

The 'git commit-graph verify' subcommand loads a commit-graph from
a given object directory instead of using the standard method
prepare_commit_graph(). During development of load_commit_graph_chain(),
a version did not include prepare_alt_odb() as it was previously
run by prepare_commit_graph() in most cases.

Add a test that prevents that mistake from happening again.

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