Can't input non ascii characters

### Checklist

- [X] I have read through the manual page (`man fzf`)
- [X] I have searched through the existing issues
- [X] For bug reports, I have checked if the bug is reproducible in the latest version of fzf

### Output of `fzf --version`

0.52.1 (6432f00)

### OS

- [ ] Linux
- [ ] macOS
- [X] Windows
- [ ] Etc.

### Shell

- [X] bash
- [ ] zsh
- [ ] fish

### Problem / Steps to reproduce

Specifically I use bash from git for windows.
OS: Win 10
Terminal:
1. Windows Terminal (1.20.11271.0)
2. mintty 3.7.0 (shipped with git for windows)

When I hit CTRL+T and try to input greek characters they appear in the prompt either transliterated to their equivalent ascii (eg `α` becomes `a`) or printed as the `?` character.
This happens with both terminals.

Both terminals are able to print greek characters (eg `ls` a folder with greek filenames) and are able to take greek input in their prompts. So the issue is with fzf's prompt.

Reproduce the prompt input problem using fzf 0.52.1 with Bash from Git for Windows on Windows 10, testing Windows Terminal and mintty with Greek characters. Trace the Ctrl+T prompt input path and verify that non-ASCII characters are preserved rather than transliterated or replaced with '?'.

Written by the indexing model from the issue text.