Add comment; improve test coverage

Author: WillAyd

cpp/src/arrow/compute/kernels/scalar_compare.cc

@@ -64,14 +64,10 @@ struct ListEqual {
     if (left.length != right.length) {
       return false;
     } else {
+      // The offsets are hard-coded to zero because the scalar unboxing and
+      // array iteration routines already set the offset from the list parent
       RangeDataEqualsImpl range_comparer{
-          EqualOptions::Defaults(),
-          false,
-          left,
-          right,
-          left.offset,
-          right.offset,
-          left.length,
+          EqualOptions::Defaults(), false, left, right, 0, 0, left.length,
       };
       return range_comparer.Compare();
     }
@@ -87,7 +83,9 @@ struct ListNotEqual {
       return true;
     } else {
       RangeDataEqualsImpl range_comparer{
-          EqualOptions::Defaults(), false, left, right, 0, 0, 1,
+          // The offsets are hard-coded to zero because the scalar unboxing and
+          // array iteration routines already set the offset from the list parent
+          EqualOptions::Defaults(), false, left, right, 0, 0, left.length,
       };
 
       return !range_comparer.Compare();

cpp/src/arrow/compute/kernels/scalar_compare_test.cc

@@ -745,7 +745,8 @@ TYPED_TEST(TestCompareList, ArrayArray) {
       {"equal", "[1, 0, 0, null]"},
       {"not_equal", "[0, 1, 1, null]"},
   };
-  const auto lhs_int = ArrayFromJSON(int_ty, R"([[1, 2, 3], [4, 5, 6], [7], null])");
+  const auto lhs_int =
+      ArrayFromJSON(int_ty, R"([[1, 2, 3], [4, 5, 6], [6, 8, 7], null])");
   const auto lhs_bin = ArrayFromJSON(
       bin_ty, R"([["a", "b", "c"], ["foo", "bar", "baz"], ["hello"], null])");
   const auto rhs_int = ArrayFromJSON(int_ty, R"([[1, 2, 3], [4, 5], [6, 7, 8], null])");