Force to recreate all diffs on import

What does this MR do?

Today, if the PullRequestImporter fails for whatever reason, for example when pulling git remote, the subsequent run will likely fail, because of ActiveRecord::RecordNotUnique. This MR makes us drop old diffs and recreate them, as method doing that suggests: #insert_or_replace.

What are the relevant issue numbers?

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/59353

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer