Fixed #24375 -- Added Migration.initial attribute

The new attribute is checked when the `migrate --fake-initial` option
is used. initial will be set to True for all initial migrations (this
is particularly useful when initial migrations are split) as well as
for squashed migrations.

tests/migrations/test_migrations/0001_initial.py

@@ -6,6 +6,8 @@ from django.db import migrations, models
 
 class Migration(migrations.Migration):
 
+    initial = True
+
     operations = [
 
         migrations.CreateModel(