On Windows, the writable temporary directory and the checkout can be on
different drives. os.path.relpath() raises ValueError for such paths before
GitConfigParser is exercised.

Skip this relative-path-specific test on Windows, where the test setup cannot
reliably construct the required relative path.

Validation:
- pytest -q test/test_config.py
- ruff check test/test_config.py
- ruff format --check test/test_config.py

Skip cross-drive relative config test on Windows

Repo.clone_from() expanded environment-variable references in caller-supplied
URLs before passing them to git clone. This could expose process environment
values to an untrusted remote and made protocol validation apply to a different
value than Git received.

Polish clone URLs without variable or home expansion on native and Cygwin
Git, then apply unsafe-protocol validation to that exact normalized value.
Preserve the literal URL when normalizing the stored origin after a successful
clone, while retaining the existing Git.polish_url() default for callers
that intentionally normalize local paths. Add regression coverage for
POSIX and Windows variable syntax, Cygwin conversion, stored origins, and
post-normalization protocol validation.

Git baseline: git clone passes URL arguments through literally; t/t5601-clone.sh
covers the accepted URL forms without shell-style environment expansion.

Security: GHSA-rwj8-pgh3-r573.

Co-authored-by: Sebastian Thiel <sebastian.thiel@icloud.com>

fix: preserve literal clone URLs