Fix a small bug in the merge schedule

A small bug related to the merge schedule and union levels was unearted after
recent changes to key generators for the state machine tests.

```
❯ cabal run lsm-tree-test -- --quickcheck-replay="(SMGen 13447116701882578385 16048829213438376903,35)"   -p '$NF=="propLockstep_RealImpl_MockFS_IO"'
lsm-tree
  Test.Database.LSMTree.StateMachine
    propLockstep_RealImpl_MockFS_IO: FAIL (28.54s)
      *** Failed! Assertion failed (after 1 test and 196 shrinks):

      ...

1 out of 1 tests failed (28.54s)
```

The bug was that the last level was always removing `Delete` entries even if
there was union level. This is now fixed.

Author: jorisdral

src/Database/LSMTree/Internal/MergeSchedule.hs

@@ -673,7 +673,7 @@ addRunToLevels tr conf@TableConfig{..} resolve hfs hbio root salt uc r0 reg leve
         traceWith tr $ AtLevel ln TraceAddLevel
         -- Make a new level
         let policyForLevel = mergePolicyForLevel confMergePolicy ln V.empty ul
-        ir <- newMerge policyForLevel MR.MergeLastLevel ln rs
+        ir <- newMerge policyForLevel (mergeTypeForLevel V.empty ul) ln rs
         pure $! V.singleton $ Level ir V.empty
     go !ln rs' (V.uncons -> Just (Level ir rs, ls)) = do
         r <- expectCompletedMerge ln ir
@@ -714,7 +714,7 @@ addRunToLevels tr conf@TableConfig{..} resolve hfs hbio root salt uc r0 reg leve
           -- Otherwise we start merging the incoming runs into the run.
           LevelLevelling -> do
             assert (V.null rs && V.null ls) $ pure ()
-            ir' <- newMerge LevelLevelling MR.MergeLastLevel ln (rs' `V.snoc` r)
+            ir' <- newMerge LevelLevelling (mergeTypeForLevel ls ul) ln (rs' `V.snoc` r)
             pure $! Level ir' V.empty `V.cons` V.empty
 
     -- Releases the incoming run.