Adjust to modified libgit2 fixture

Author: carlosmn

test/reference_test.rb

@@ -26,6 +26,7 @@ def test_list_references
     assert_equal [
       "refs/heads/br2",
       "refs/heads/dir",
+      "refs/heads/executable",
       "refs/heads/ident",
       "refs/heads/long-file-name",
       "refs/heads/master",