#3865 followup for fix performance degression introduced by switching order of operands (#4284)

Summary:
Followup for #4266. There is one more place in **get_context.cc** where **MergeOperator::ShouldMerge** should be called with reversed list of operands.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4284

Differential Revision: D9380008

Pulled By: sagar0

fbshipit-source-id: 70ec26e607e5b88465e1acbdcd6c6171bd76b9f2

table/get_context.cc

@@ -262,7 +262,7 @@ bool GetContext::SaveValue(const ParsedInternalKey& parsed_key,
           merge_context_->PushOperand(value, false);
         }
         if (merge_operator_ != nullptr &&
-            merge_operator_->ShouldMerge(merge_context_->GetOperands())) {
+            merge_operator_->ShouldMerge(merge_context_->GetOperandsDirectionBackward())) {
           state_ = kFound;
           if (LIKELY(pinnable_val_ != nullptr)) {
             Status merge_status = MergeHelper::TimedFullMerge(