tail: improve throughput for -c N file

[oech3]

Part of #11607 (non-stream)
The code path without + is different.

$ target/debug/tail -c 1PB /tmp/huge | pv>/dev/null
^C.9GiB 0:00:01 [39.9GiB/s]
$ tail -c 1PB /tmp/huge | pv>/dev/null
^C02GiB 0:00:01 [2.02GiB/s]

[oech3]

$ truncate -s 48k /tmp/page
$ taskset -c 0 hyperfine -N "target/release/tail -c 48k /tmp/page" "target/release/tail-splice -c 48k /tmp/page"
Benchmark 1: target/release/tail -c 48k /tmp/page
  Time (mean ± σ):       1.4 ms ±   0.2 ms    [User: 0.5 ms, System: 0.8 ms]
  Range (min … max):     1.1 ms …   4.0 ms    1836 runs
 
Benchmark 2: target/release/tail-splice -c 48k /tmp/page
  Time (mean ± σ):       1.3 ms ±   0.2 ms    [User: 0.5 ms, System: 0.7 ms]
  Range (min … max):     1.1 ms …   4.6 ms    1660 runs
 
Summary
  target/release/tail-splice -c 48k /tmp/page ran
    1.01 ± 0.25 times faster than target/release/tail -c 48k /tmp/page

Less than the size is very subtle. 32kib sometimes wins and loses.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 309 untouched benchmarks
⏩ 46 skipped benchmarks¹

---

_{Comparing oech3:tail-non-plus (d6fbfa0) with main (30fd234)}

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. ↩

[xtqqczze]

1.01 ± 0.25 times faster than target/release/tail -c 48k /tmp/page

It's not statistically significant?