Spell self.Index as self.INDEX in IndexFile.diff

Because, as noted in 65863a2, it is now (slightly) preferable to
write INDEX. Note that this is solely stylstic: they are equivalent
and must remain so, so that existing code that uses GitPython and
accesses it as Index (including in overridden diff methods when
subclassing Diffable) continues to work.

This changes it in the exception message as well.

Author: EliahKagan

Diff for: git/index/base.py

@@ -1500,7 +1500,7 @@ def diff(
         if self._file_path != self._index_path():
             raise AssertionError("Cannot call %r on indices that do not represent the default git index" % self.diff())
         # Index against index is always empty.
-        if other is self.Index:
+        if other is self.INDEX:
             return git_diff.DiffIndex()
 
         # Index against anything but None is a reverse diff with the respective item.
@@ -1514,12 +1514,12 @@ def diff(
             # Invert the existing R flag.
             cur_val = kwargs.get("R", False)
             kwargs["R"] = not cur_val
-            return other.diff(self.Index, paths, create_patch, **kwargs)
+            return other.diff(self.INDEX, paths, create_patch, **kwargs)
         # END diff against other item handling
 
         # If other is not None here, something is wrong.
         if other is not None:
-            raise ValueError("other must be None, Diffable.Index, a Tree or Commit, was %r" % other)
+            raise ValueError("other must be None, Diffable.INDEX, a Tree or Commit, was %r" % other)
 
         # Diff against working copy - can be handled by superclass natively.
         return super().diff(other, paths, create_patch, **kwargs)