Bugfix: Restore old behavior of `snapshots --latest <n>` without `--group-by`

Restic 0.19.0 accidentally changed the behavior of `snapshots --latest <n>`
to no longer group snapshots by host and paths by default.

The `snapshots --latest <n>` command now again uses the old behavior of
grouping by host and paths when `--group-by` is not specified. However, when
specifying `--group-by` the output is still grouped as requested, as in restic
0.19.0.

https://github.com/restic/restic/issues/21869
https://github.com/restic/restic/pull/21875
