Fixes #2530.

Summary

This fixes a Windows path handling error when --config points to a file on a different drive from the repository root.

os.path.relpath raises ValueError for paths on different Windows drives, so this keeps the original absolute path when a relative path cannot be computed.

Tests

Added a Windows regression test for --config on another drive.

Validation run locally:

python -m pytest tests/main_test.py -k different_drive
python -m pytest tests/main_test.py -k adjust_args_and_chdir
python -m pytest tests/main_test.py
python -m py_compile pre_commit/main.py tests/main_test.py
git diff --check