Use FugitiveWorkTree() to find the git root

Fix #1586

Author: junegunn

autoload/fzf/vim.vim

@@ -715,6 +715,9 @@ endfunction
 " ------------------------------------------------------------------
 
 function! s:get_git_root(dir)
+  if empty(a:dir) && exists('*FugitiveWorkTree')
+    return FugitiveWorkTree()
+  endif
   let dir = len(a:dir) ? a:dir : substitute(split(expand('%:p:h'), '[/\\]\.git\([/\\]\|$\)')[0], '^fugitive://', '', '')
   let root = systemlist('git -C ' . shellescape(dir) . ' rev-parse --show-toplevel')[0]
   return v:shell_error ? '' : (len(a:dir) ? fnamemodify(a:dir, ':p') : root)