Working with WriteBatchWithIndex and merge

For the background story - I am trying to work with `WBWI` (`WriteBatchWithIndex`) and merge operations. I have a need to be able to see the `merged` value of a key in the `WBWI` after I have performed the merge operation, e.g.

``` C++
db->Put(key, val)

...

wbwi->Merge(key, merge_val);

auto dbIt = db->NewIterator();
auto it = wbwi->NewIteratorWithBase(dbIt);

for (it.Seek(keyPrefix); it.isValid() && memcmp(iterator.key(), keyPrefix) > 0; it.Next()) {
// unable to read the merged value here
it.Value();
}
```

It seems that `WBWIterator` does not respect the merge semantics, which I really need and would have expected it to do intrinsically.

So... I thought I would try and understand `WBWI::GetFromBatch` and `WBWI::GetFromBatchAndDB` to see if I could use those as an alternative.

However, I am confused around the `rocksdb::Status` meaning of `kMergeInProgress` for those functions.

For example,in `utilities/write_batch_with_index/write_batch_with_index_test.cc` when looking in at the test `TestGetFromBatchMerge`, I don't understand why the status for `x` after calling `GetFromBatch` is `OK`:

``` C++
batch.Put("x", "X");
...
batch.Merge("x", ToString(i));
...
s = batch.GetFromBatch(column_family, options, "x", &value);
ASSERT_OK(s);
```

Yet, when compared to `TestGetFromBatchMerge2`, after calling `Merge` the status is `kMergeInProgress`, e.g.

``` C++
batch.Put(column_family, "X", "x");
s = batch.GetFromBatch(column_family, options, "X", &value);
ASSERT_OK(s);
ASSERT_EQ("x", value);

batch.Put(column_family, "X", "x2");
s = batch.GetFromBatch(column_family, options, "X", &value);
ASSERT_OK(s);
ASSERT_EQ("x2", value);

batch.Merge(column_family, "X", "aaa");
s = batch.GetFromBatch(column_family, options, "X", &value);
ASSERT_TRUE(s.IsMergeInProgress());
```

The code comments in `write_batch_with_index.h` are a bit vague. I am wondering under what circumstances when calling `GetFromBatch` (and `GetFromBatchAndDB`) will the status be `kMergeInProgress`?