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

@@ -97,31 +97,31 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   def diffs
     apply_diff_view_cookie!
 
-    @merge_request_diff =
-      if params[:diff_id]
-        @merge_request.merge_request_diffs.viewable.find(params[:diff_id])
-      else
-        @merge_request.merge_request_diff
-      end
-
-    @merge_request_diffs = @merge_request.merge_request_diffs.viewable.select_without_diff
-    @comparable_diffs = @merge_request_diffs.select { |diff| diff.id < @merge_request_diff.id }
-
-    if params[:start_sha].present?
-      @start_sha = params[:start_sha]
-      @start_version = @comparable_diffs.find { |diff| diff.head_commit_sha == @start_sha }
-
-      unless @start_version
-        @start_sha = @merge_request_diff.head_commit_sha
-        @start_version = @merge_request_diff
-      end
-    end
-
-    @environment = @merge_request.environments_for(current_user).last
-
     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])
+          else
+            @merge_request.merge_request_diff
+          end
+
+        @merge_request_diffs = @merge_request.merge_request_diffs.viewable.select_without_diff
+        @comparable_diffs = @merge_request_diffs.select { |diff| diff.id < @merge_request_diff.id }
+
+        if params[:start_sha].present?
+          @start_sha = params[:start_sha]
+          @start_version = @comparable_diffs.find { |diff| diff.head_commit_sha == @start_sha }
+
+          unless @start_version
+            @start_sha = @merge_request_diff.head_commit_sha
+            @start_version = @merge_request_diff
+          end
+        end
+
+        @environment = @merge_request.environments_for(current_user).last
+
         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: