commit-graph: test --split across alternate without --split

We allow sharing commit-graph files across alternates. When we are writing a split commit-graph, we allow adding tip graph files that are not in the alternate, but include commits from our local repo. However, if our alternate is not using the split commit-graph format, its file is at .git/objects/info/commit-graph and we are trying to write files in .git/objects/info/commit-graphs/graph-{hash}.graph. We already have logic to ensure we do not merge across alternate boundaries, but we also cannot have a commit-graph chain to our alternate if uses the old filename structure. Create a test that verifies we create a new split commit-graph with only one level and we do not modify the existing commit-graph in the alternate. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>