Use fzf#shellescape instead (#1503)

doc/fzf-vim.txt

@@ -375,7 +375,7 @@ predefined `Ag` or `Rg` using `fzf#vim#grep`.
 >
     command! -bang -nargs=* GGrep
       \ call fzf#vim#grep(
-      \   'git grep --line-number -- '.shellescape(<q-args>),
+      \   'git grep --line-number -- '.fzf#shellescape(<q-args>),
       \   fzf#vim#with_preview({'dir': systemlist('git rev-parse --show-toplevel')[0]}), <bang>0)
 <
                                                            {11} bin/preview.sh