Prompt user again at ambiguos input

Take for example the following prompt:

```
[A]pply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort?
```

If I now enter `bm` and press enter, beets ignores the second character and aborts (`b`). I would expect beets to ask me again, as it can not be determined if I wanted beets to abort (`b`) or to show me more candidates (`m`).

Reproduce the ambiguous CLI prompt with `bm` and inspect the input-handling entry point that interprets the choices. Confirm how single-character commands are matched and add coverage for ambiguous input if the existing tests cover this prompt. Done means `bm` does not abort immediately and the user is prompted again.

Written by the indexing model from the issue text.