sort/compare on string does not work correctly when another sort is used within the compare function

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Can be demonstrated with code like:

s1: sort/compare "abcd" func[a b][s2: sort/compare/reverse "1234" func[a b][a < b] a < b]
s1 ;== "dcab" ;;<--- should be "abcd"
s2 ;== "4321" ;;<--- OK

or:

s1: sort/compare "abcdabcd" func[a b][s2: sort/compare "áéíáéíáéí" func[a b][a < b] a < b]
s1 ;== "ddbacabc" ;;<--- should be "aabbccdd"
s2 ;== "áááéééííí" ;;<--- OK

Related to #1100