patch 8.0.1361: some users don't want to diff with hidden buffers

Problem:    Some users don't want to diff with hidden buffers.
Solution:   Add the "hiddenoff" item to 'diffopt'. (Alisue, closes #2394)
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 3273dcb..d6c5c83 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -375,6 +375,29 @@
   %bwipe
 endfunc
 
+func Test_diffopt_hiddenoff()
+  set diffopt=filler,foldcolumn:0,hiddenoff
+  e! one
+  call setline(1, ['Two', 'Three'])
+  redraw
+  let normattr = screenattr(1, 1)
+  diffthis
+  botright vert new two
+  call setline(1, ['One', 'Four'])
+  diffthis
+  redraw
+  call assert_notequal(normattr, screenattr(1, 1))
+  set hidden
+  close
+  redraw
+  " should not diffing with hidden buffer two while 'hiddenoff' is enabled
+  call assert_equal(normattr, screenattr(1, 1))
+
+  bwipe!
+  bwipe!
+  set hidden& diffopt&
+endfunc
+
 func Test_diffoff_hidden()
   set diffopt=filler,foldcolumn:0
   e! one