patch 9.0.1309: scrolling two lines with even line count and 'scrolloff' set

Problem:    Scrolling two lines with even line count and 'scrolloff' set.
Solution:   Adjust how the topline is computed. (closes #10545)
diff --git a/src/testdir/test_scrollbind.vim b/src/testdir/test_scrollbind.vim
index 8074fda..211b05e 100644
--- a/src/testdir/test_scrollbind.vim
+++ b/src/testdir/test_scrollbind.vim
@@ -218,8 +218,8 @@
 	      \ '7 line 02 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ 02',
 	      \ '56789ABCDEFGHIJKLMNOPQRSTUVWXYZ 02',
 	      \ 'UTSRQPONMLKJIHGREDCBA9876543210 02',
-	      \ '. line 11 ZYXWVUTSRQPONMLKJIHGREDCBA9876543210 11',
-	      \ '. line 11 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ 11',
+	      \ '. line 10 ZYXWVUTSRQPONMLKJIHGREDCBA9876543210 10',
+	      \ '. line 10 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ 10',
 	      \ ''],  getline(1, '$'))
   enew!