Revert "Use TreeMap in MergeUsing for safety" and add a comment

This reverts commit 2cdafd3a.

h2/src/main/org/h2/command/dml/MergeUsing.java

@@ -6,8 +6,8 @@
 package org.h2.command.dml;
 
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.HashSet;
-import java.util.TreeMap;
 
 import org.h2.api.ErrorCode;
 import org.h2.api.Trigger;
@@ -278,7 +278,11 @@ public class MergeUsing extends Prepared {
     private String queryAlias;
     private int countUpdatedRows;
     private Select targetMatchQuery;
-    private final TreeMap<Value, Integer> targetRowidsRemembered;
+    /**
+     * Contains mappings between _ROWID_ and ROW_NUMBER for processed rows. Row
+     * identities are remembered to prevent duplicate updates of the same row.
+     */
+    private final HashMap<Value, Integer> targetRowidsRemembered = new HashMap<>();
     private int sourceQueryRowNumber;
 
 
@@ -286,7 +290,6 @@ public class MergeUsing extends Prepared {
         super(session);
         this.targetTable = targetTableFilter.getTable();
         this.targetTableFilter = targetTableFilter;
-        targetRowidsRemembered = new TreeMap<>(session.getDatabase().getCompareMode());
     }
 
     @Override