detect when stat(2)-ing all files is not needed
For plain file listing, no stat(2) information is needed; inspect the format string and ordering options to ensure this is true. We assume any custom test implies needing to stat for now. recurse detects in opendir/readdir whether to recurse on DT_DIR etc. This massively speeds up listing large trees, making lr -U almost as fast as plain GNU find(1).
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