detecting cycles in Git's commit graph

Are there any tools for detecting cycles in the commit graph which have been caused by grafts? I thought 'git fsck' might do it, but it doesn't seem to.

While importing some historic code into a new Git repository, my import scripts accidentally created a cycle in the commit graph by using grafts. Essentially, I had commits like:

    A -- B -- C

I accidentally created a graft marking commit C as commit B's second parent. There were 15,000 grafts so it took me a while to track down the one causing the cycle. The initial symptom was that 'git log --graph B' produced no output.

I wasn't able to find any existing tools to locate these kinds of faulty grafts.

Thanks for any pointers.