Fix commits list 500 with multi-file editor new_repo cookie

What does this MR do?

Fix commits list 500 with multi-file editor new_repo cookie, page /<namespace>/<project>/commmits/master

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

When the new_repo cookie was set, visiting the commit list page would result in a 500.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #39821 (closed)