Use appendStringInfoString instead of appendStringInfo where possible.

This shaves a few cycles, and generally seems like good programming
practice.

David Rowley

src/backend/access/rmgrdesc/clogdesc.c

@@ -37,5 +37,5 @@ clog_desc(StringInfo buf, uint8 xl_info, char *rec)
 		appendStringInfo(buf, "truncate before: %d", pageno);
 	}
 	else
-		appendStringInfo(buf, "UNKNOWN");
+		appendStringInfoString(buf, "UNKNOWN");
 }