Fix create_patch with mnemonic diff prefixes

[KSHITIZ6341]

Summary

• override diff.mnemonicPrefix for Git commands whose output GitPython parses
• cover both the shared Diffable.diff path and the index-versus-empty-tree implementation
• add regression coverage for commit-to-working-tree, index-to-working-tree, and index-to-empty-tree patch parsing

Fixes #2013.

Root cause

With diff.mnemonicPrefix=true in Git configuration, Git emits prefixes such as c/, i/, and w/. GitPython's patch parser expects the standard a/ and b/ prefixes, so create_patch=True silently returned an empty DiffIndex.

User impact

Repository or global mnemonic-prefix preferences no longer change the machine-readable patch format used internally by GitPython. Patch-producing diff calls return the expected entries without requiring callers to override their Git configuration.

Validation

• GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master .venv/bin/pytest test/test_diff.py --no-cov (23 passed)
• uvx ruff check git/diff.py git/index/base.py test/test_diff.py
• python -m compileall -q git test/test_diff.py
• git diff --check

AI disclosure

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