Support index diffs against the empty tree

[puneetdixit200]

Related to #2025.

IndexFile.diff() is documented as accepting the same targets as Diffable.diff(), including NULL_TREE, but the index-specific implementation rejected NULL_TREE before it could produce a diff. That leaves no direct API path for inspecting staged entries for an initial commit against the empty tree.

This adds explicit IndexFile.diff(NULL_TREE) handling using git diff --cached against the empty tree object. I kept IndexFile.diff(None) as the existing index-vs-working-tree comparison, since that behavior is documented in the quick docs and existing tests.

The regression test creates a new repository, stages a file, writes the index, constructs a fresh IndexFile, and verifies that index.diff(NULL_TREE) reports the staged addition while index.diff(None) remains empty because the working tree matches the index.

Validation:

• .venv/bin/python -m pytest --no-cov test/test_index.py::TestIndex::test_index_file_diff_null_tree_with_initial_index -q
• .venv/bin/python -m ruff check git/index/base.py test/test_index.py

AI assistance disclosure: OpenAI GPT-5 helped draft and check this change; I reviewed the implementation and tests before submitting.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds IndexFile.diff(NULL_TREE) support so callers can diff the index against an “empty tree” (useful for initial index contents), and introduces a regression test to validate expected diff output.

Changes:

• Extends IndexFile.diff() to accept and handle NULL_TREE as the other diff target.
• Implements a custom git diff --cached <empty-tree> invocation to produce raw/patch output for the NULL_TREE case.
• Adds a test covering initial index diffing against NULL_TREE, including path filtering and patch creation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
test/test_index.py	Adds coverage for diffing an initial index against NULL_TREE and verifying raw + patch output.
git/index/base.py	Implements IndexFile.diff(NULL_TREE) by invoking git diff --cached against the empty-tree SHA.

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[puneetdixit200]

AI agent on behalf of Puneet: pushed fe4b66d to use a shared empty-tree SHA constant, accept that SHA for index diffs, and make the existing empty-index assertion explicit.

Verified:

• .venv/bin/python -m pytest test/test_index.py::TestIndex::test_index_file_diff_null_tree_with_initial_index -q
• .venv/bin/python -m ruff check git/diff.py git/index/base.py test/test_index.py
• git diff --check

[Byron]

It seems bots are running around and fixing to dos automatically. And in a way, it's fine. This seems to be addressing the TODO legitimately and the bots already hashed it out, so I'll just try rolling with it.

The biggest problem GitPython has is that I don't think it has a human who knows what good Python is supposed to be like these days. So, agents will basically rule this place and all I can do is to apply high level quality constraints - better than nothing.