lib.fileset: Add benchmark for unions

Changed files
+4
lib
fileset
+4
lib/fileset/benchmark.sh
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bench 'toSource { root = ./.; fileset = ./.; }'
rm -rf -- *
+
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touch {0..1000}
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bench 'toSource { root = ./.; fileset = unions (mapAttrsToList (name: value: ./. + "/${name}") (builtins.readDir ./.)); }'
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rm -rf -- *