[2.1.x] Fixed #29749 -- Made the migrations loader ignore files starting with a tilde or underscore.

Regression in 29150d5da8.
Backport of 32fbccab40 from master

tests/migrations/test_loader.py

@@ -500,6 +500,14 @@ class LoaderTests(TestCase):
         }
         self.assertEqual(plan, expected_plan)
 
+    @override_settings(MIGRATION_MODULES={'migrations': 'migrations.test_migrations_private'})
+    def test_ignore_files(self):
+        """Files prefixed with underscore, tilde, or dot aren't loaded."""
+        loader = MigrationLoader(connection)
+        loader.load_disk()
+        migrations = [name for app, name in loader.disk_migrations if app == 'migrations']
+        self.assertEqual(migrations, ['0001_initial'])
+
 
 class PycLoaderTests(MigrationTestBase):