[Merge operand meant for key K is being applied on wrong key]

Summary:
We iterate until we find a different key than original key.
ikey is pointing to next key when we break out of loop.
After the loop we apply all merge operands meant for original key
on the next key!

Test Plan:
Need to give a build to Marcin to test out.

Revert Plan: OK

Task ID: #3181932

Reviewers: haobo, emayanke, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14073

db/merge_helper.cc

@@ -175,7 +175,7 @@ void MergeHelper::MergeUntil(Iterator* iter, SequenceNumber stop_before,
     assert(kTypeMerge == orig_ikey.type);
     assert(operands_.size() >= 1);
     assert(operands_.size() == keys_.size());
-    success_ = user_merge_operator_->FullMerge(ikey.user_key, nullptr,
+    success_ = user_merge_operator_->FullMerge(orig_ikey.user_key, nullptr,
                                                operands_, &merge_result,
                                                logger_);