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.