Use standard prefixes for parsed patch diffs

[pick7]

Summary

• Force diff.mnemonicPrefix=false only for patch output parsed by GitPython.
• Cover the shared Diffable path and IndexFile's empty-tree path.
• Verify the temporary Git option does not affect a following command.

Fixes #2013.

Validation

• .venv/bin/python -m pytest -o addopts= test/test_diff.py::TestDiff::test_patch_diff_ignores_mnemonic_prefix
• .venv/bin/python -m ruff check git/diff.py git/index/base.py test/test_diff.py
• .venv/bin/python -m ruff format --check git/diff.py git/index/base.py test/test_diff.py
• .venv/bin/python -m mypy git/diff.py git/index/base.py

Note: the complete test/test_diff.py run has one pre-existing failure in TestDiff.test_diff_with_staged_file on this local Git version; the focused regression passes.

AI agent disclosure

This pull request was prepared and submitted by OpenAI Codex acting as an AI agent through the contributor account.

[Copilot]

Pull request overview

This PR makes GitPython’s create_patch=True diff parsing robust against user Git configuration diff.mnemonicPrefix=true by forcing the standard a/ and b/ prefixes only for the patch output that GitPython parses into Diff objects.

Changes:

• Apply a temporary -c diff.mnemonicPrefix=false git-option for patch-formatted diffs generated by the shared Diffable.diff() path.
• Apply the same temporary option for IndexFile.diff() when diffing against the empty tree (NULL_TREE path).
• Add a regression test asserting patch diffs parse correctly under diff.mnemonicPrefix=true and that the config remains unchanged for subsequent commands.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/test_diff.py	Adds regression coverage for parsing patch diffs when diff.mnemonicPrefix=true, including NULL_TREE and ensuring no option leakage.
git/index/base.py	Forces standard prefixes for parsed patch output in the index’s empty-tree diff path.
git/diff.py	Forces standard prefixes for parsed patch output in the shared Diffable.diff() implementation when create_patch=True.

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[Byron]

Thanks, LGTM.