Resolve "Define /commits/ behaviour"

What does this MR do?

Redirects namespace_id/:project_id/commits/ to namespace_id/:project_id/commits/:root_ref

I added a method to the ApplicationsController which is used by the route commits_root or namespace_id/:project_id/commits/.

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

Why was this MR needed?

To fix #31576 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #31576 (closed)