`cargo crev fetch` does not work with ssh URLs

I am unable to use `crev fetch` with `git@` (ssh) URLs.

```bash
> RUST_BACKTRACE=1 cargo crev fetch url git@github.com:dpc/crev-proofs.git
Fetching git@github.com:dpc/crev-proofs.git... Error: authentication required but no callback set; class=Ssh (23)
```

Using the same `fetch` command with an `https` URL works.

`crev` was installed using `cargo`; I am running on Mac OS X Mojave (10.14.6).

```
> cargo crev --version
cargo-crev-crev 0.9.0
```

Start by reproducing `cargo crev fetch url git@github.com:dpc/crev-proofs.git` with `RUST_BACKTRACE=1`, then compare it with the working HTTPS command. Trace the `cargo crev fetch` entry point and its Git SSH authentication path; done means SSH URLs fetch successfully without the authentication callback error.

Written by the indexing model from the issue text.