grep: short-circuit inverted empty patterns

[wondr-wclabs]

Closes #27.

Summary

• detect the -v + empty-pattern case after regex compilation, before stdin/file traversal
• return exit status 1 without count output or file-open errors when no line can be selected
• add regression coverage for count mode, missing input, and invalid-regex ordering

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing wondr-wclabs:codex/empty-invert-short-circuit (360f8a5) with main (d28bf76)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[sylvestre]

please add a comment explaining why

[wondr-wclabs]

Added a comment at the branch explaining the behavior: an empty pattern matches every line, so with -v GNU selects no lines and returns exit 1 without scanning inputs.

I kept that check before creating the searcher so missing files are not touched in this case; the direct regression check is grep -v "" missing-file, which now exits 1 with no stdout/stderr.

[sylvestre]

do we really need a function here ?

[wondr-wclabs]

No, it was not pulling its weight. I removed the helper and inlined the condition next to the early return.

That keeps the special-case logic visible with the explanatory comment, and avoids a one-use function whose name still required jumping back to understand why the early exit is GNU-compatible.

[sylvestre]

see the comments

[lhecker]

Oh interesting! Thanks!