du: Reuse existing metadata instead of calling path.is_dir() again

[jesseschalken]

Path::is_dir does another call to stat but we can reuse the metadata we already have.

Also Path::is_dir traverses symbolic links which is only right if -L was passed.

~1.13x speedup on my test directory.

Before

> cargo build --release
> measure-command { .\target\release\coreutils.exe du --apparent-size "test folder" }
TotalSeconds      : 11.0101422

After

> cargo build --release
> measure-command { .\target\release\coreutils.exe du --apparent-size "test folder" }
TotalSeconds      : 9.7109226

[sylvestre]

nice could you please try to benchmark with hyperfine ? and document your work in src/uu/du/BENCHMARKING.md ?

see other commands for example

[sylvestre]

and i wonder if we could automate this check to make sure we don't regress :)

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/rm/rm1 (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[sylvestre]

ping ?