Enhancement: Support JSON output and quiet mode for the `diff` command

The `diff` command now supports outputting machine-readable output in JSON
format. To enable this, pass the `--json` option to the command. To only print
the summary and suppress detailed output, pass the `--quiet` option.

https://github.com/restic/restic/issues/2508
https://github.com/restic/restic/pull/3592
