patch 9.0.0114: the command line takes up space even when not used

Problem:    The command line takes up space even when not used.
Solution:   Allow for 'cmdheight' to be set to zero. (Shougo Matsushita,
            closes #10675, closes #940)
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 47988b2..cfa5b63 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -1481,9 +1481,12 @@
     call assert_equal(h0, winheight(0))
     call assert_equal(1, &cmdheight)
   endfor
+  " supports cmdheight=0
+  set cmdheight=0
   call assert_true(win_move_statusline(0, 1))
-  call assert_equal(h0, winheight(0))
-  call assert_equal(1, &cmdheight)
+  call assert_equal(h0 + 1, winheight(0))
+  call assert_equal(0, &cmdheight)
+  set cmdheight&
   " check win_move_statusline from bottom window on top window ID
   let id = win_getid(1)
   for offset in range(5)