Keep a terminal newline as part of a gitignore filename

[kokotatan]

Summary

A gitignore rule such as foo currently matches a path named foo\n in the simple and Hyperscan backends, although Git keeps the newline as part of the filename. Python's $ anchor may match before a final newline; Hyperscan has the same behavior for $ and \Z.

Generate Python's strict \Z end anchor, and translate it to the equivalent \z spelling before compiling RE2/Hyperscan expressions. Keep escaped literal backslash-Z filenames intact and preserve the directory-marker splitting logic. RE2's existing split $ already has strict end-of-text semantics. The same translation keeps a custom RegexPattern using a Python \Z anchor consistent across backends.

The change is distinct from merged #139 (globstars containing newlines in parent directories) and open #150 (trimming whitespace from pattern lines). This fixes matching of filenames that themselves end with a newline.

Validation

• New public regression tests fail on the original upstream code: 40 failing subtests, no test errors. Tests cover both pattern classes, string/byte compiled patterns, all three backends, negation, and escaped literal backslashes.
• Full unittest suite: 227 tests passed, 14 platform skips, with RE2 and Hyperscan installed alongside the simple backend on CPython 3.12.12.
• Independent git check-ignore --stdin -z oracle: 480 comparisons across ten pattern configurations and three backends. Mismatches decrease from 46 to 8; all 38 corrected cases concern terminal-newline handling, and there are zero new mismatches. The eight remaining simple-backend ancestor-negation mismatches are identical on untouched upstream and remain outside this change.
• mypy pathspec tests/check_usage.py and pyright pathspec tests/check_usage.py report the same 5 and 8 diagnostics, respectively, on this patch and an untouched upstream checkout. These concern missing RE2 typing and existing _typing.py/usage-check diagnostics; no new diagnostic is introduced.
• Sphinx documentation builds with warnings as errors; git diff --check passes.

AI assistance: OpenAI Codex prepared and validated the patch and tests. Claude Opus independently reviewed the relevant source and backend contracts before submission. No human review is claimed.