Bugfix: Avoid choosing parent snapshots newer than time of new snapshot

The `backup` command, when a `--parent` was not provided, previously chose the
most recent matching snapshot as the parent snapshot. However, this didn't make
sense when the user passed `--time` to create a new snapshot older than the most
recent snapshot.

Instead, `backup` now chooses the most recent snapshot which is not newer than
the snapshot-being-created's timestamp, to avoid any time travel.

https://github.com/restic/restic/pull/3619
