Fixed #22135 -- Added ModelAdmin.get_changeform_initial_data().

Allows custom behavior for setting initial form data in ModelAdmin.
By default, initial data is set via GET params. The new method allows
this behavior to be overridden.

Thanks egasimus for the suggestion.

docs/ref/contrib/admin/index.txt

@@ -1709,6 +1709,21 @@ templates used by the :class:`ModelAdmin` views:
     ``obj_display`` is a string with the name of the deleted
     object.
 
+.. method:: ModelAdmin.get_changeform_initial_data(request)
+
+    .. versionadded:: 1.7
+
+    A hook for the initial data on admin change forms. By default, fields are
+    given initial values from ``GET`` parameters. For instance,
+    ``?name=initial_value`` will set the ``name`` field's initial value to be
+    ``initial_value``.
+
+    This method should return a dictionary in the form
+    ``{'fieldname': 'fieldval'}``::
+
+        def get_changeform_initial_data(self, request):
+            return {'name': 'custom_initial_value'}
+
 Other methods
 ~~~~~~~~~~~~~