uucore: Disallow slashes in determine_backup_suffix #11149

Performance Gate Passed

⚡ 1 improved benchmark
✅ 293 untouched benchmarks
⏩ 42 skipped benchmarks¹

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
⚡	Simulation	`shuf_input_range[1000000]`	89.7 ms	84.8 ms	+5.76%

_{Comparing Zellic:backupcontrol-no-path-traversal (ae6ecb1) with main (a611f98)}

42 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. ↩