cargo: bump gix to 0.67.0 (includes minor behavior change)

- gix::object::tree::diff::change::Event::Rewrite is flattened
- diff options are extracted to separate type
  GitoxideLabs/gitoxide@2b81e6c
- signature text now includes a trailing newline
  GitoxideLabs/gitoxide@4a6bbb1

The last change means that our SecureSig { sig } returned by GitBackend is now
terminated by '\n'. I think this is harmless since textual signature is usually
ends with '\n'.