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)
diff --git a/src/version.c b/src/version.c
index af3baf5..3682a45 100644
--- a/src/version.c
+++ b/src/version.c
@@ -779,6 +779,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1547,
+/**/
     1546,
 /**/
     1545,