Shallow clone fails on merge commits

[Pan-Maciek]

How to reproduce the issue:

1. Create a merge request based on main branch with one commit or more
2. Do at least one commit to main
3. Clone repo with depth 1 and single branch main

repo, err := git.PlainClone(RepositoryPath, false, &git.CloneOptions{
  URL:           url,
  SingleBranch:  true,
  Tags:          git.NoTags,
  ReferenceName: "main",
  Depth:         1,
})

5. Fetch changes

err := repo.Fetch(&git.FetchOptions{
  Tags:     git.NoTags,
  Force:    true,
})

What happens

• git merge commit contains two parents
• second parent has history older than grafted commit so at some point when traversing commits you get object that don't exist

How git cli solves this issue?

First commit that is unrechable from second parent points to grafted commit. At lest that's what I would assume from reproducing this scenario with git cli client.