Inconsistent merge request comment import for Gitlab Migration

Summary

Gitlab migration has inconsistent import of merge request comments and merge request status.

• Event type comments like added commit are missing
• Merge request status is sometimes inconsistent

Steps to reproduce

• Issue can be reproduced by running migration of project: https://staging.gitlab.com/gitlab-migration/dri
• Example of nightly pipeline which is reproducing the issue: https://gitlab.com/gitlab-org/manage/import/import-github-performance/-/jobs/2321876665

Example Project

• https://staging.gitlab.com/gitlab-migration/dri
• 2022-04-07_11-11-417_gitlab-migration_dri_export.tar.gz

What is the current bug behavior?

• Missing comments which are imported correctly by file based importer
• Incorrect merge request status

What is the expected correct behavior?

All data migrated correctly

Relevant logs and/or screenshots

{"feature_category":"importers","severity":"ERROR","time":"2022-04-12T07:40:16.383Z","correlation_id":"fcc73b45291503364fe35244419248aa","bulk_import_id":1,"bulk_import_entity_id":2,"bulk_import_entity_type":"project_entity","pipeline_class":"BulkImports::Projects::Pipelines::MergeRequestsPipeline","pipeline_step":"loader","exception_class":"ActiveRecord::RecordNotUnique","exception_message":"PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint \"index_merge_requests_on_target_project_id_and_iid\"\nDETAIL:  Key (target_project_id, iid)=(2, 8) already exists.\n"},
{"feature_category":"importers","severity":"ERROR","time":"2022-04-12T07:40:18.007Z","correlation_id":"fcc73b45291503364fe35244419248aa","bulk_import_id":1,"bulk_import_entity_id":2,"bulk_import_entity_type":"project_entity","pipeline_class":"BulkImports::Projects::Pipelines::MergeRequestsPipeline","pipeline_step":"loader","exception_class":"DiffNote::NoteDiffFileCreationError","exception_message":"Failed to find diff file"}