GitLab Migration - Migrate Commit discussions
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## Proposed Solution
* For each migrated project, migrate [discussions attached to commits](https://docs.gitlab.com/ee/user/discussions/#add-a-comment-to-a-commit).
## Documentation
Following documentation should be updated with the objects/fields added in this issue:
* [ ] User docs: https://docs.gitlab.com/ee/user/group/import/
* [ ] Dev docs: https://docs.gitlab.com/ee/development/bulk_import.html
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