diffutils: fix panic on non-UTF-8 argument ending in --width=N

[SAY-5]

diff panicked with exit 101 on a non-UTF-8 argument whose lossy form ends in --width=N, e.g. diff $'\xff--width=5' A B. The --width regex was missing the leading ^ anchor that --tabsize already has, so the lossy (U+FFFD) form matched and the following into_string().unwrap() then failed on the real bytes.

Anchoring the regex (^--width=...$, matching tabsize_re) makes such an argument fall through to the operand path instead of being parsed as a width option, so it no longer panics. As a side effect diff xyz--width=5 is now also treated as a filename rather than silently accepting a width.

Added a width test and a #[cfg(unix)] regression test for the non-UTF-8 case.

Fixes #247.

[codspeed-hq]

Merging this PR will degrade performance by 3.03%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 4 regressed benchmarks
✅ 21 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cmd_cmp_gnu_equal[1000]	12.2 µs	13.1 µs	-6.35%
❌	Simulation	cmd_cmp_release_equal[25000]	13 µs	13.7 µs	-5.76%
❌	Simulation	cmd_cmp_release_equal[100]	13.2 µs	13.8 µs	-4.16%
❌	Simulation	cmd_cmp_release_equal[10000]	13 µs	13.5 µs	-3.74%
⚡	Simulation	cmd_diff_release_equal[1000]	13.5 µs	12.9 µs	+5.28%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing SAY-5:fix-width-non-utf8-panic (8fdc882) with main (dc9ca17)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (dc9ca17) to head (8fdc882).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #250   +/-   ##
===========================
===========================

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