Only load what we need when visiting MR diffs page

The instance variables set in this action are only used by the JSON
response. Visiting `merge_requests/:iid/diffs` in a browser always loads
the `show` action, so we don't need to find an MR diff, or an
environment, in that case.

app/controllers/projects/merge_requests_controller.rb

@@ -97,6 +97,9 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   def diffs
     apply_diff_view_cookie!
 
+    respond_to do |format|
+      format.html { define_discussion_vars }
+      format.json do
         @merge_request_diff =
           if params[:diff_id]
             @merge_request.merge_request_diffs.viewable.find(params[:diff_id])
@@ -119,9 +122,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
 
         @environment = @merge_request.environments_for(current_user).last
 
-    respond_to do |format|
-      format.html { define_discussion_vars }
-      format.json do
         if @start_sha
           compared_diff_version
         else

changelogs/unreleased/mr-diffs-speed-up.yml

+---
+title: Speed up initial rendering of MR diffs page
+merge_request:
+author: