vim-patch:8.1.2289: after :diffsplit closing the window does not disable diff

Problem:    After :diffsplit closing the window does not disable diff.
Solution:   Add "closeoff" to 'diffopt' and add it to the default.
c823477979

src/nvim/options.lua

@@ -612,7 +612,7 @@ return {
       alloced=true,
       redraw={'current_window'},
       varname='p_dip',
-      defaults={if_true={vi="internal,filler"}}
+      defaults={if_true={vi="internal,filler,closeoff"}}
     },
     {
       full_name='digraph', abbreviation='dg',