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

`restic backup` (if a `--parent` is 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 snapshot older than the most recent snapshot.

Instead, `restic 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