artists: don't show unchanged URLs in artist history.

Only show changed URLs in the artist history, not unchanged URLs. Makes
the edit history more compact and easier to read by showing only the
things that changed on every edit.

app/views/application/_diff_list.html.erb

@@ -15,7 +15,9 @@
     <% end %>
   <% end %>
 
-  <% diff.unchanged.each do |item| %>
-    <%= tag.li item, class: ["unchanged", *li_class] %>
+  <% if show_unchanged %>
+    <% diff.unchanged.each do |item| %>
+      <%= tag.li item, class: ["unchanged", *li_class] %>
+    <% end %>
   <% end %>
 <% end %>