Freeze if running :@ while opening a file

- [x] I have fzf 0.23.0 or above
- [X] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [X] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [X] I have read through the manual page of fzf (`man fzf`)
- [X] I have searched through the existing issues

Vim freezes if using fzf.vim to open a file that runs `:@` (see `:h :@`). E.g.,
```(sh)
$ echo 1 > t.t
$ vim -c 'au BufRead t.t let @o = "echom 111\nechom 222" | @o'
:Files
t.t
(freeze)
```
, but does not freeze if opened with `:e` instead of `:Files`. This used to work without problems, don't know why it doesn't work now.

Edit: `:Files!` (fullscreen) works fine. So my workaround is
```(vim)
let g:fzf_layout = exists('$TMUX') ? {'tmux': '-p90%,80%'} : {}
```

Reproduce the freeze with the supplied Vim command, the :Files picker, and a BufRead autocommand that executes :@; compare it with :e and the working :Files! path. Inspect the fzf.vim entry points for file opening and the interaction with Vim's :@ handling. Done means opening the file through :Files no longer freezes, while the reported command still runs correctly.

Written by the indexing model from the issue text.