Need to press Enter twice on Windows

- [x] I have read through the manual page (`man fzf`)
- [x] I have the latest version of fzf
- [x] I have searched through the existing issues

## Info

- OS
- [ ] Linux
- [ ] Mac OS X
- [x] Windows
- [ ] Etc.
- Shell
- [x] bash
- [ ] zsh
- [ ] fish
- Vim: Neovim 0.5

## Problem / Steps to reproduce

When I use a function like `fzf#run(fzf#wrap({}))`, when I select an option, the first time I press Enter the fzf process receives it and empties the terminal correctly, but the terminal popup window does not exit. It stays empty and open. I need to press Enter again to make it go away and actually navigate to the file I want.

I suspect this is because of `cmd.exe`'s `/C` flag requiring one more "enter" before it ends the cmd process, as this issue does not happen on Linux.

A probable fix would be to run the executable yourself instead of relying on the shell to parse the command line.