mv: symlink exponential walk fix

[mtvb]

scan_directory_recursive decides recursion with path.is_dir(), which follows symlinks.
Two ln -s .. symlinks branch *2 per level down to the kernel's 40-symlink ELOOP limit, creating 2^40 paths.

Fix: Decide recursion from symlink_metadata() instead of path.is_dir(), so the scanner never descends through a symlink. Since the else branch already called symlink_metadata(), it means no extra syscall - the metadata is now fetched once and used for both the recursion decision and the grouping check.

Fixes #14285

[sylvestre]

entry.file_type() answers the is-a-directory question straight from readdir's d_type with no syscall on Linux; you only need symlink_metadata() on the non-directory branch for nlink/dev/ino.

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 152.20 MB (+1016 KB, +0.66%)

Significant per-binary changes:
  comm     1.12 MB ->    2.33 MB  (+1.21 MB, +107.29%)

[mtvb]

I was unsure about the is_file() branch cause one of my tests failed. My test was faulty, entry.metadata() is fine for files.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/tail-n0f is now passing!