fzf.vim cannot be used on Windows inside git-bash shell. [Interested to push a PR, but I need many directions]

- [x] I have fzf 0.30.0 or above
- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [ ] 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

I have been using fzf successfully under Ubuntu bash shell.

But there are several issues when trying to use in with git-bash shell on Windows 10.

**edit:** Using Neovim version 0.8.0-1210-gd367ed9b2

## Without `set shell`
If I do not set the shell in my `init.vim`, the shell is:

shell="C:\Program Files\Git\usr\bin\bash.exe"

Then I cannot get any `fzf` command to execute. E.g.

:Files

fails with error:

> Error detected while processing function fzf#vim#files[15]..94_fzf[1]..94_check_requirements[11]..fzf#exec[31]..function fzf#vim#files[15]..94_fzf[1]..94_check_requirements[11]..fzf#exec:
> line 31:
> E605: Exception not caught: Failed to run "C:\Users\adnn\AppData\Local\nvim-data\plugged\fzf/bin/fzf --version"

## With `set shell=cmd`

`fzf` commands `:Files` and `:Rg` work, as they show the pop-up window that is searchable. But the preview does not work:

> bash: C:UsersadnAppDataLocalNVIM-D~1pluggedfzf.vimbinpreview.sh: No such file or directory

`:Buffers` does not work at all. There is an error showing in the pop-up window

> [Command failed: type ^"C:\Users\adn\AppData\Local\Temp\nvim.0\FCdV6q\4^"]

---

I would be interested to address these issues, but I never worked with vim script, nor this project sources, so I would need directions.
I am most interested to address the first case (without setting a shell, which then uses git's `bash.exe`). From the error, I suppose the problem is that `bash` fails to execute `fzf - -version` because the absolute path is starting with `C:\Users`, which is not a valid git-bash path.

Any pointer where this path is generated in the plugin code?