grep: -B retains exactly N context lines, not next power of two

[sylvestre]

No description provided.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing sylvestre:fix-before-context-count (404a68a) with main (670669e)}

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. ↩