And the winner is...[JGit](https://eclipse.googlesource.com/jgit/jgit/+/2fbf296fda205446eac11a13abd4fcdb182f28d9).

Note the delta cycle behavior mentioned in that commit...I believe we are still susceptible to it. It may be worth adding a check to `index-pack` to reject them (it should be trivial to detect the multiple entries when we are generating the index).