Command to generate debug report

There are cases where users end up in a bad state with git-spice, e.g. a rebase loop as reproduced in #582.
To debug these, they have to try to recall the steps they took, their repository state, etc. before we can reproduce the error.

We should add a command that generates a report of the git-spice state and recent changes to it so they can include it in bug reports to ease reproduction and debugging; we already have the information in refs/spice/data history and current state of the repository.