fix: use shared empty tree sha for index diffs

Assisted-by: ChatGPT

Author: puneetdixit200

git/diff.py

@@ -3,7 +3,7 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
-__all__ = ["DiffConstants", "NULL_TREE", "INDEX", "Diffable", "DiffIndex", "Diff"]
+__all__ = ["DiffConstants", "NULL_TREE", "NULL_TREE_SHA", "INDEX", "Diffable", "DiffIndex", "Diff"]
 
 import enum
 import re
@@ -84,6 +84,9 @@ class DiffConstants(enum.Enum):
 :const:`git.NULL_TREE` and :const:`Diffable.NULL_TREE`.
 """
 
+NULL_TREE_SHA = "4b825dc642cb6eb9a060e54bf8d69288fbee4904"
+"""SHA of Git's canonical empty tree object."""
+
 INDEX: Literal[DiffConstants.INDEX] = DiffConstants.INDEX
 """Stand-in indicating you want to diff against the index.
 

git/index/base.py

@@ -1511,10 +1511,10 @@ def diff(
         if other is self.INDEX:
             return git_diff.DiffIndex()
 
-        if other is git_diff.NULL_TREE:
+        if other == git_diff.NULL_TREE or other == git_diff.NULL_TREE_SHA:
             args: List[Union[PathLike, str]] = [
                 "--cached",
-                "4b825dc642cb6eb9a060e54bf8d69288fbee4904",
+                git_diff.NULL_TREE_SHA,
                 "--abbrev=40",
                 "--full-index",
             ]

test/test_index.py

@@ -24,6 +24,7 @@
 import pytest
 
 from git import BlobFilter, Diff, Git, IndexFile, NULL_TREE, Object, Repo, Tree
+from git.diff import NULL_TREE_SHA
 from git.exc import (
     CheckoutError,
     GitCommandError,
@@ -568,7 +569,7 @@ def test_index_file_diff_null_tree_with_initial_index(self, rw_dir):
         index.write()
 
         index = IndexFile(repo)
-        assert not index.diff(None)
+        self.assertEqual(len(index.diff(None)), 0)
 
         diff = index.diff(NULL_TREE)
         self.assertEqual(len(diff), 1)
@@ -577,6 +578,7 @@ def test_index_file_diff_null_tree_with_initial_index(self, rw_dir):
         self.assertEqual(diff[0].b_path, filename)
 
         self.assertEqual(len(index.diff(NULL_TREE, paths=filename)), 1)
+        self.assertEqual(len(index.diff(NULL_TREE_SHA, paths=filename)), 1)
         self.assertEqual(len(index.diff(NULL_TREE, paths="missing")), 0)
 
         patch = index.diff(NULL_TREE, create_patch=True)