Broken commit history display if there's a visible merge commit

[taneliang]

If there's a merge commit above the earliest interesting commit, our frontend vomits blood.

Steps to repro

1. Clone repro repo: https://github.com/24r/stack-attack-issue-44-repro. These are the branches:
   • main: the main branch, which includes a merge commit
   • eic: a branch pointing to the earliest interesting commit. This branch exists so that the interesting commit graph will contain a merge commit. This also means that if you delete this branch, Stack Attack will display the commit graph correctly.
   • c: just a random feature branch
2. See Git commit graph:
   
3. Launch Stack Attack
4. See the commit graph. The merge commit and everything above it are duplicated:
   

Steps to resolve

• Confirm that GSC is producing the commit graph correctly (it probably is). If it is, this bug is in the frontend
• Decide how we want to solve this bug. Displaying a proper commit graph like Git may be too difficult as we'll need to implement a proper topographical sort of the commit history. We could try to find another way to display merge commits, e.g. maybe only showing merge commits when hasFork in displayCommitsForSubgraphRootedAtCommit is false.