Dispatch needed JS when creating a new MR in diff view

What does this MR do?

• Dispatch the needed JS code for the new_diffs action in the MR controller when creating a new MR
• Set @show_changes_tab variable if location is new/diffs

Why was this MR needed?

• Both WIP messages show if you
  • create a new MR
  • select the Changes tab
  • refresh the page

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27920 (closed)