mv: use GNU default error when overwriting non-empty directory

[aaron-ang]

Closes #5102.

The GNU default mv error message: cannot overwrite {target}: Directory not empty, is clear enough for the case of overwriting non-empty directory. It clearly states the overwrite intent and that target is the empty directory.

Our current message: cannot move {source} to {target}: Directory not empty, is causing confusion (see #5102 (comment)) and should be reverted.

In addition, I made test cases stricter by expecting stderr only and replaced some comments with full expected errors.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/mv/dir2dir. tests/mv/dir2dir is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/mv/dir2dir. tests/mv/dir2dir is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/sort/sort-debug-keys. tests/sort/sort-debug-keys is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/env/env-signal-handler is now passing!

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing aaron-ang:mv-msg (0121dce) with main (d2bd2d3)}

Summary

✅ 142 untouched benchmarks
⏩ 180 skipped benchmarks¹

Footnotes

1. 180 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. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)

[aaron-ang]

#10435 should fix the CI errors.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)

[aaron-ang]

hi @sylvestre, could you please take a look at this?