truncate: simplify fn is_modifier

[oech3]

No description provided.

[codspeed-hq]

Merging this PR will improve performance by 10.65%

⚡ 2 improved benchmarks
✅ 337 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	cp_recursive_deep_tree[(120, 4)]	699.2 KB	615.6 KB	+13.59%
⚡	Simulation	complex_relative_date	148 µs	137.3 µs	+7.78%

Tip

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_{Comparing oech3:trunc-cont (76d935e) with main (bb71d03)}

Footnotes

1. 46 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. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.

[cakebaker]

Thanks!