local: stop --copy-links following symlink loops - fixes #4402

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With -L/--copy-links a symlink pointing to one of its parent
directories was followed until the OS gave up with "too many levels
of symbolic links" 40 levels deep. As every looping symlink multiplies
the listing, a small tree with three such symlinks listed millions of
entries.

When a followed symlink points to a directory, compare it with the
directory being listed and each of its parents using os.SameFile and
if it matches report an error and skip it, the same way as a circular
symlink. A symlink to a sibling directory is still followed. Loops
excluded by the directory filters are skipped quietly as before.

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