Simplify parse representation of savepoint commands

Instead of embedding the savepoint name in a list and then requiring
complex code to unpack it, just add another struct field to store it
directly.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>

src/backend/nodes/copyfuncs.c

@@ -3602,6 +3602,7 @@ _copyTransactionStmt(const TransactionStmt *from)
 
 	COPY_SCALAR_FIELD(kind);
 	COPY_NODE_FIELD(options);
+	COPY_STRING_FIELD(savepoint_name);
 	COPY_STRING_FIELD(gid);
 
 	return newnode;