Fix diff and commit view on merge requests

with new sidebar
Fixes #3981

app/helpers/nav_helper.rb

@@ -32,8 +32,10 @@ def page_sidebar_class
   end
 
   def page_gutter_class
-
-    if current_path?('merge_requests#show') || current_path?('issues#show')
+    if current_path?('merge_requests#show') || 
+      current_path?('merge_requests#diffs') || 
+      current_path?('merge_requests#commits') || 
+      current_path?('issues#show')
       if cookies[:collapsed_gutter] == 'true'
         "page-gutter right-sidebar-collapsed"
       else