Wrapping `anyhow::Error` in another error loses backtrace

Migrating from `failure` to `anyhow`, I encounter an issue where I start losing backtraces when `anyhow::Error`s are wrapped in another error type:

```rust
#[test]
fn backtrace_preserved_after_thiserror_derive() {
#[derive(Debug, thiserror::Error)]
pub enum SomeError {
#[error(transparent)]
Anyhow(anyhow::Error),
}

let mut errs = (0..2).map(|_| anyhow::anyhow!("aaa"));

let wrapped = anyhow::Error::from(SomeError::Anyhow(errs.next().unwrap()));
let notwrapped = errs.next().unwrap();

assert_eq!(wrapped.backtrace().to_string(), notwrapped.backtrace().to_string());
}
```
It is a kind of usage suggested by the documentation:
https://github.com/dtolnay/thiserror/blob/799b3d33c4e302d20023ea47b524be2c97f03ac5/src/lib.rs#L194-L195
It used to be possible to work around this issue in `failure` (even on stable, which we are on).
It would seem fine if the backtrace could only be propagated if coming from `anyhow`.

Is it possible to achieve something like this with the `anyhow`/`thiserror` combination?