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/equalfuncs.c

@@ -1513,6 +1513,7 @@ _equalTransactionStmt(const TransactionStmt *a, const TransactionStmt *b)
 {
 	COMPARE_SCALAR_FIELD(kind);
 	COMPARE_NODE_FIELD(options);
+	COMPARE_STRING_FIELD(savepoint_name);
 	COMPARE_STRING_FIELD(gid);
 
 	return true;