fix(git): keep git status --porcelain empty on a clean tree

[asem89]

Summary

• rtk git status --porcelain prints ok on a clean tree instead of the zero bytes git emits.
• For --porcelain, --porcelain=v1/v2, -z and --null, emptiness is the result — consumers parse "no output" as "no changes", so the ok compaction breaks them.
• filter_status_with_args now takes the user's args and skips the substitution for those four machine-readable formats. Human-readable output, including short -s, is unchanged.

Why it bites

The standard clean-tree check inverts:

$ cd /tmp/clean-repo
$ git status --porcelain | wc -c
0
$ rtk git status --porcelain | wc -c
3          # "ok"

$ [ -z "$(rtk git status --porcelain)" ] && echo clean || echo dirty
dirty      # on a clean tree

--porcelain isn't routed through the compact path — uses_compact_status_path returns false for it — so it reaches filter_status_with_args and gets rewritten.

Test plan

• cargo fmt --all --check && cargo clippy --all-targets -- -D warnings && cargo test --all — 2657 passed, 0 failed
• Two regression tests added: test_filter_status_porcelain_clean_stays_empty (all four flags) and test_filter_status_short_still_collapses_to_ok (guards the compaction that should stay)
• Manual, against a clean throwaway repo: rtk git status --porcelain → 0 bytes; rtk git status → still clean — nothing to commit

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[asem89]

Closing — I'm not in a position to sign the CLA, so this can't be merged as-is. Leaving the analysis here in case a maintainer wants to reimplement it; both are small and self-contained, and the reproduction steps in the description stand on their own. No response needed.