Enhancement: Support JSON output and quiet mode for diff

We've added support for getting machine-readable output for snapshot diff, just pass the
flag `--json` for `restic diff` and restic will output a JSON-encoded diff stats and change
list.

Passing the `--quiet` flag to the `diff` command will only print the summary
and suppress the detailed output.

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