unexpand: use buffered read

[cerdelen]

Changing from using read_until to read() and necessary additions to the unexpand_line function to keep behaviour.

Fix: #10698

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will improve performance by 34.66%

_{Comparing cerdelen:unexpand_buffer (f1f58b8) with main (dc982b1)}

Summary

⚡ 2 improved benchmarks
✅ 282 untouched benchmarks
⏩ 38 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	unexpand_many_lines[100000]	263.6 ms	195.8 ms	+34.64%
⚡	Simulation	unexpand_large_file[10]	552.6 ms	410.4 ms	+34.66%

Footnotes

1. 38 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. ↩