Test that redefined Diffable.Index should be compatible

This tests the most important characteristics of Diffable.Index
that must be (and are) preserved in making it an alias of the
enumeration constant INDEX instead of a nested class definition.

Adding this additional test also allows commit.INDEX to be used in
place of commit.Index in the existing test (since part of what this
tests is that they are the same object). That change is also made,
along with some very minor cleanup of immediately nearby test code.

Author: EliahKagan

Diff for: test/test_diff.py

@@ -12,7 +12,7 @@
 import ddt
 import pytest
 
-from git import NULL_TREE, Diff, DiffIndex, GitCommandError, Repo, Submodule
+from git import NULL_TREE, Diff, DiffIndex, Diffable, GitCommandError, Repo, Submodule
 from git.cmd import Git
 from test.lib import StringProcessAdapter, TestBase, fixture, with_rw_directory
 
@@ -352,15 +352,15 @@ def test_diff_submodule(self):
         self.assertIsInstance(diff.b_blob.size, int)
 
     def test_diff_interface(self):
-        # Test a few variations of the main diff routine.
+        """Test a few variations of the main diff routine."""
         assertion_map = {}
         for i, commit in enumerate(self.rorepo.iter_commits("0.1.6", max_count=2)):
             diff_item = commit
             if i % 2 == 0:
                 diff_item = commit.tree
             # END use tree every second item
 
-            for other in (None, NULL_TREE, commit.Index, commit.parents[0]):
+            for other in (None, NULL_TREE, commit.INDEX, commit.parents[0]):
                 for paths in (None, "CHANGES", ("CHANGES", "lib")):
                     for create_patch in range(2):
                         diff_index = diff_item.diff(other=other, paths=paths, create_patch=create_patch)
@@ -406,10 +406,22 @@ def test_diff_interface(self):
         diff_index = c.diff(cp, ["does/not/exist"])
         self.assertEqual(len(diff_index), 0)
 
+    def test_diff_interface_stability(self):
+        """Test that the Diffable.Index redefinition should not break compatibility."""
+        self.assertIs(
+            Diffable.Index,
+            Diffable.INDEX,
+            "The old and new class attribute names must be aliases.",
+        )
+        self.assertIs(
+            type(Diffable.INDEX).__eq__,
+            object.__eq__,
+            "Equality comparison must be reference-based.",
+        )
+
     @with_rw_directory
     def test_rename_override(self, rw_dir):
-        """Test disabling of diff rename detection"""
-
+        """Test disabling of diff rename detection."""
         # Create and commit file_a.txt.
         repo = Repo.init(rw_dir)
         file_a = osp.join(rw_dir, "file_a.txt")