vim-patch:8.1.1769: 'shellslash' is also used for completion

Problem:    'shellslash' is also used for completion.
Solution:   Add the 'completeslash' option. (Yasuhiro Matsumoto, closes vim/vim#3612)
ac3150d385

src/nvim/options.lua

@@ -452,6 +452,15 @@ return {
       varname='p_cot',
       defaults={if_true={vi="menu,preview"}}
     },
+    {
+      full_name='completeslash', abbreviation='csl',
+      type='string', scope={'buffer'},
+      vi_def=true,
+      vim=true,
+      varname='p_csl',
+      enable_if='BACKSLASH_IN_FILENAME',
+      defaults={if_true={vi=""}}
+    },
     {
       full_name='confirm', abbreviation='cf',
       type='bool', scope={'global'},