Fix lua completions having off-by-one error in neovim 0.9+ (gelguy#187)

A private method vim._expand_pat (which should not be used) has
changed the behavior, which causes an off-by-one error in completion.

As a temporary workaround, we manually offset the match position.

autoload/wilder/cmdline.vim

@@ -1222,6 +1222,9 @@ function! s:get_lua_completion(ctx, res, fuzzy) abort
     let l:arg_pos = len(a:res.arg)
   else
     let [l:candidates, l:arg_pos] = luaeval('{vim._expand_pat("^" .. _A[1])}', [a:res.arg])
+    if has('nvim-0.9.0') && l:arg_pos > 0
+        let l:arg_pos = l:arg_pos - 1   " see wilder.nvim#187, neovim#23363
+    endif
 
     if a:fuzzy
       let l:last_char = a:res.arg[-1 :]