add git integration

[mathpn]

• Feature Addition: Introduced Git integration to the llm-docsmith tool, enabling it to update docstrings only for functions and classes modified since the last commit.
• Efficiency Improvement: Optimized docstring updates for large codebases by targeting only changed code, reducing unnecessary processing.
• Documentation Update: Enhanced the README.md with usage instructions for the new Git integration flags (--git and --git-base).
• Core Logic Updates: Modified docsmith.py to implement the new behavior, including changes to handle Git-based file comparisons.
• Testing: Added a comprehensive test suite (tests/test_docsmith.py) to validate the new Git integration functionality.
• Version Increment: Updated pyproject.toml to reflect a new version (0.2) aligned with the feature release.