patch 8.2.1211: removed more than dead code

Problem:    Removed more than dead code.
Solution:   Put back the decrement.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index a3f6d58..c0c5d0d 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -1130,4 +1130,24 @@
   redraw
 endfunc
 
+func Test_diff_and_scroll()
+  " this was causing an ml_get error
+  set ls=2
+  for i in range(winheight(0) * 2) 
+    call setline(i, i < winheight(0) - 10 ? i : i + 10) 
+  endfor
+  vnew
+  for i in range(winheight(0)*2 + 10) 
+    call setline(i, i < winheight(0) - 10 ? 0 : i) 
+  endfor
+  diffthis
+  wincmd p
+  diffthis
+  execute 'normal ' . winheight(0) . "\<C-d>"
+
+  bwipe!
+  bwipe!
+  set ls&
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab