Use documented verion_info in test_index_file_diffing

Instead of directly accessing the _version_info attribute.

This fixes a new test failure since the way the public version_info
property uses the _version_info backing attribute has changed, and
also shows the usage that is documented (accessing _version_info
was never guaranteed to work, and now longer works as before).

Author: EliahKagan

Diff for: test/test_index.py

@@ -528,7 +528,7 @@ def test_index_file_diffing(self, rw_repo):
             index.checkout(test_file)
         except CheckoutError as e:
             # Detailed exceptions are only possible in older git versions.
-            if rw_repo.git._version_info < (2, 29):
+            if rw_repo.git.version_info < (2, 29):
                 self.assertEqual(len(e.failed_files), 1)
                 self.assertEqual(e.failed_files[0], osp.basename(test_file))
                 self.assertEqual(len(e.failed_files), len(e.failed_reasons))