Diff support

Be able to provide a diff of a given dependency between versions. Something like:

```
cargo depdiff -d name-of-dependency main..this-branch
```

And if the dependency changed from version X to Y, it would print the diff between its versions X and Y.

Start by tracing the existing `cargo depdiff` command and how it identifies dependency versions; the issue provides the proposed `-d name-of-dependency main..this-branch` entry point. Define how the selected dependency and its versions are resolved, then verify that the command reports the diff between versions X and Y when the dependency changes.

Written by the indexing model from the issue text.