Fixed #26117 -- Consulted database routers in initial migration detection.

Thanks Simon Charette for help.

tests/migrations/routers.py

@@ -0,0 +1,9 @@
+class TestRouter(object):
+    def allow_migrate(self, db, app_label, model_name=None, **hints):
+        """
+        The Tribble model should be the only one to appear in the 'other' db.
+        """
+        if model_name == 'tribble':
+            return db == 'other'
+        elif db == 'other':
+            return False