Preserve Git filenames in added-file and case checks

[xiehuanyi]

With Git's default core.quotePath=true, check-added-large-files --maxkb=1 mañana.txt returns success for a staged 2 KiB file. The staged path is escaped by Git, so it no longer matches the filename supplied to the hook. The same mismatch lets check-case-conflict miss CAFÉ.py when café.py is already committed.

Read the staged-file and repository-file lists with -z and the existing zsplit helper. This preserves the actual paths, including non-ASCII characters, tabs, and newlines.

Validation:

• Seven new real-Git regression cases fail on unchanged main.
• The complete Python 3.11 suite passes: 420 tests, four existing expected failures, 100% coverage.
• All configured checks pass for the changed files, including mypy.

The tests set core.quotePath=true explicitly. Filenames containing tabs or newlines are skipped on Windows, where those names are invalid; the Unicode cases run on all platforms. This was found during source review of Git filename handling. AI assistance: OpenAI gpt-6.1-sol, xhigh, through Codex.

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