Avoid choosing parent snapshot newer than time of current snapshot

Currently, `restic backup` (if a `--parent` is not provided)
will choose the most recent matching snapshot as the parent snapshot.
This makes sense in the usual case,
where we tag the snapshot-being-created with the current time.

However, this doesn't make sense if the user has passed `--time`
and is currently creating a snapshot older than the latest snapshot.
Instead, choose the most recent snapshot
which is not newer than the snapshot-being-created's timestamp,
to avoid any time travel.

Impetus for this change:
I'm using restic for the first time!
I have a number of existing BTRFS snapshots
I am backing up via restic to serve as my initial set of backups.
I initially `restic backup`'d the most recent snapshot to test,
then started backing up each of the other snapshots.
I noticed in `restic cat snapshot <id>` output
that all the remaining snapshots have the most recent as the parent.

cmd/restic/cmd_restore.go

@@ -118,7 +118,7 @@ func runRestore(opts RestoreOptions, gopts GlobalOptions, args []string) error {
 	var id restic.ID
 
 	if snapshotIDString == "latest" {
-		id, err = restic.FindLatestSnapshot(ctx, repo, opts.Paths, opts.Tags, opts.Hosts)
+		id, err = restic.FindLatestSnapshot(ctx, repo, opts.Paths, opts.Tags, opts.Hosts, nil)
 		if err != nil {
 			Exitf(1, "latest snapshot for criteria not found: %v Paths:%v Hosts:%v", err, opts.Paths, opts.Hosts)
 		}