Fix issue with iterator operations in this order: Prev(), Seek(), Prev()

Summary:
Due to a bad merge of D14163 and D14001 before checking in D14001, "direction_ = kForward;" in MergeIterator::Seek() was deleted my mistake (in commit b135d01e7b ). It will generate wrong results or assert failure after the sequence of Prev() (or SeekToLast()), Seek() and Prev().

Fix it

Test Plan: make all check

Reviewers: igor, haobo, dhruba

Reviewed By: igor

CC: yhchiang, i.am.jin.lei, ljin, leveldb

Differential Revision: https://reviews.facebook.net/D16527

table/merger.cc

@@ -122,6 +122,7 @@ class MergingIterator : public Iterator {
       // one, or null if there is no first child.
       current_ = first_child;
     }
+    direction_ = kForward;
   }
 
   virtual void Next() {
@@ -228,6 +229,8 @@ class MergingIterator : public Iterator {
   // If the value is true, both of iterators in the heap and current_
   // contain valid rows. If it is false, only current_ can possibly contain
   // valid rows.
+  // This flag is always true for reverse direction, as we always use heap for
+  // the reverse iterating case.
   bool use_heap_;
   Env* const env_;
   // Which direction is the iterator moving?