tee: merge flush into write for simplicity

[oech3]

Then we can avoid let res = (|| { *** })();.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/bounded-memory is now passing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[xtqqczze]

I don't see the benefit of this change, it also means we won't be able to extract Writer to uucore::writable.

[oech3]

I believe that there is an advantage to do that more than moving this to uucore since

• posix tee requirement is specialized compared with other utils
• let res = ... is nightmare when I call write_all at another place

[oech3]

I guess we already have same things on comm and unexpand we can move to uucore.

[codspeed-hq]

Merging this PR will degrade performance by 3.13%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 310 untouched benchmarks
⏩ 46 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	du_max_depth_balanced_tree[(6, 4, 10)]	25.1 ms	25.9 ms	-3.13%

---

_{Comparing oech3:tee-flush (d756c7a) with main (5316f58)}

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]

I guess we already have same things on comm and unexpand we can move to uucore.

We could use Writer in place of Box<dyn Write> elsewhere to avoid dynamic dispatch.

[oech3]

tee does not require fn write anyway. So we don't need those code on tee.rs.
We can recreate this at uucore if needed.

[xtqqczze]

It doesn't make sense to replace idiomatic Write implementation just to avoid one instance of let res = (|| { *** })();.

[oech3]

I'm planning to use it and really want to avoid to put the large block on the main logic.

[oech3]

Also this is the only way to avoid executing 2 match.