Enhancement: support creating snapshot summary statistics for old snapshots

When `rewrite` is used with the `--snapshot-summary` option, a new snapshot is
created containing statistics summary data. Only two fields in the summary will
be non-zero: `TotalFilesProcessed` and `TotalBytesProcessed`.

When rewrite is called with one of the `--exclude` options, `TotalFilesProcessed`
and `TotalBytesProcessed` will be updated in the snapshot summary.

https://github.com/restic/restic/issues/4942
https://github.com/restic/restic/pull/5185