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_scroll_opt.vim b/src/testdir/test_scroll_opt.vim
index c146c01..01b1daf 100644
--- a/src/testdir/test_scroll_opt.vim
+++ b/src/testdir/test_scroll_opt.vim
@@ -38,6 +38,23 @@
   quit!
 endfunc
 
+func Test_scolloff_even_line_count()
+   new
+   resize 6
+   setlocal scrolloff=3
+   call setline(1, range(20))
+   normal 2j
+   call assert_equal(1, getwininfo(win_getid())[0].topline)
+   normal j
+   call assert_equal(1, getwininfo(win_getid())[0].topline)
+   normal j
+   call assert_equal(2, getwininfo(win_getid())[0].topline)
+   normal j
+   call assert_equal(3, getwininfo(win_getid())[0].topline)
+
+   bwipe!
+endfunc
+
 func Test_CtrlE_CtrlY_stop_at_end()
   enew
   call setline(1, ['one', 'two'])