title: In ModelAdmin, how do I get the object's previous values when overriding `save_model()`
body: <p>When overriding <code>ModelAdmin.save_model()</code>, I want to be able to run some calculations between the object's new values vs its old ones. Is there any way that I can get the "old object" with all its previous data before the change?</p>
  
  <p>For example, if I have an <code>Object</code> with <code>obj.name</code> = "foo" that I update via the Django admin app to now be <code>obj.name</code> = "bar", upon saving the following code should print out accordingly:</p>
  
  <pre><code>from django.contrib import admin
  
  class ObjectAdmin(admin.ModelAdmin):
       def save_model(self, request, obj, form, change):
            old_object = self.get_old_object()
            print(old_object.name)  # Should print out "foo"
            print(obj.name)         # Should print out "bar"
  </code></pre>