Provide a diff of the archives when the archive or checksum of a package is modified

[kit-ty-kate]

Changes of archive and/or their checksums happens from time to time on opam-repository. It is usually related to how github creates those archives, but sometimes, a mistake can be made and the archive can change in an unexpected way (e.g. ocaml/opam-repository#15294 (comment)).

I think it would be good to have Camelus give a diff when they change to avoid mistakes in the future.

cc @thomasblanc

[thomasblanc]

Same as #35, I'll keep you posted.

(because of other projects, It may take some time before I can go deep into it).