patch 8.0.1547: undo in the options window makes it empty

Problem:    Undo in the options window makes it empty.
Solution:   Set 'undolevels' while filling the buffer. (Yasuhiro Matthew,
            closes #2645)

src/version.c

@@ -778,6 +778,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1547,
 /**/
     1546,
 /**/