fix(git): preserve status trailing newline

[nightcityblade]

Summary

• Preserve the trailing newline for explicit git status --short and --porcelain output.
• Add a regression test covering empty, terminated, and unterminated status output.

Fixes #3303

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: compared native and RTK git status --short byte-for-byte; output ends in 0a.

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