[GFiles] Avoid duplicate entries (#473)

It is presumed that duplicates in `git ls-files` are contiguous.

autoload/fzf/vim.vim

@@ -511,7 +511,7 @@ function! fzf#vim#gitfiles(args, ...)
   endif
   if a:args != '?'
     return s:fzf('gfiles', {
-    \ 'source':  'git ls-files '.a:args,
+    \ 'source':  'git ls-files '.a:args.(s:is_win ? '' : ' | uniq'),
     \ 'dir':     root,
     \ 'options': '-m --prompt "GitFiles> "'
     \}, a:000)