Error: The repo at path ... is unusable due to having an invalid HEAD reference

When I run `cargo-upgrade upgrade` I get:

```
Error: The repo at path /home/gabor/.cargo/registry/index/github.com-1ecc6299db9ec823 is unusable
due to having an invalid HEAD reference: reference 'refs/heads/main' not found; class=Reference (4);
code=NotFound (-3)
```

Apparently this was caused by me Ctrl-C-in a previous run of `cargo-upgrade upgrade` that seemed to have been stuck (not showing anything for more than a minute) After removing that folder and running `cargo-upgrade upgrade` again (and Ctrl-C-ing again) I get the same error.

Start with the `cargo-upgrade upgrade` entry point and reproduce the failure by interrupting a run, then inspect how the cargo registry index is handled afterward. Done means an interrupted upgrade does not leave the index unusable or cause subsequent upgrades to fail with an invalid HEAD reference.

Written by the indexing model from the issue text.