getcwd() is not correct after using :FZF

- [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
- [x] Linux
- [ ] Mac OS X
- [ ] Windows
- [ ] Etc.
- Shell
- [x] bash
- [ ] zsh
- [ ] fish

## Problem / Steps to reproduce

In vim, after calling `:FZF` with `autochdir` set, `echo getcwd()` gives the original directory (or root directory of a git repo if called inside of a git repo) instead of the one containing the selected file. `:pwd` does update to the new directory.

1. Open vim
2. Run `:set autochdir`
3. Run `:FZF` and select a file in a different directory
4. `:echo getcwd()` shows the original directory
5. `:pwd` correctly shows the parent of the selected file

I made the following change in plugin/fzf.vim which is working for me so far but is probably an ugly hack:

```
@@ -254,6 +254,13 @@ function! s:common_sink(action, lines) abort
finally
let &autochdir = autochdir
silent! autocmd! fzf_swap
+ if autochdir
+ let new_path = getcwd() . "/" . item
+ if !isdirectory(new_path)
+ let new_path = fnamemodify(new_path, ":h:p")
+ endif
+ exe "lcd " . new_path
+ endif
endtry
```

This problem leads to issues with other plugins like the one mentioned in https://github.com/junegunn/fzf.vim/issues/993