Create new get-pattern-vars fn for SPARQL MINUS

[quoll]

The SPARQL MINUS operation requires that outer vars that don't appear in patterns (EPVPatterns) should not be included in bindings. This means that the get-vars function should not be recursing into FILTER or BINDING. That will require a new operation, which is essentially the same as the existing one, but which returns nothing for filters.

asami/src/asami/query.cljc

Line 298 in 3609943

vars (reduce #(into %1 (map plain-var (get-vars %2))) #{} patterns)