Using `@merge_request` in Rapid Diffs provides an incomplete MergeRequest object

The following discussion from !180675 (merged) should be addressed:

• @jerasmus started a discussion: (+2 comments)

  thought (non-blocking): I wonder if can can set metadata_endpoint here now that !183795 (merged) is merged 🤔

  [ LATER ]

  @jerasmus I tried diff_files_metadata_project_merge_request_path(@project, @merge_request) here and it errored:

  No route matches {
      :action=>"diff_files_metadata",
      :controller=>"projects/merge_requests",
      :id=>#<MergeRequest id: thomasrandolph/work!>,
      :namespace_id=>#<Namespaces::UserNamespace id:94 @thomasrandolph>,
      :project_id=>#<Project id:19 thomasrandolph/work>>,
      :tab=>"diffs"
  }, possible unmatched constraints: [:id]

  Looks like maybe the ID of the @merge_request is incorrect (incomplete), so I'll need someone with back end knowledge to dig into why our merge request is broken. In the mean time, this is broadly out of scope (sort of) for the actual task here, so I think we can probably proceed and update it in a follow-up.