Change: Ignore parent snapshot for `backup --stdin`

Restic uses a parent snapshot to speed up directory scanning when performing
backups, but this only wasted time and memory when the backup source is stdin
(using the `--stdin` option of the `backup` command), since no directory scanning
is performed in this case.

Snapshots made with `backup --stdin` no longer have a parent snapshot, which allows
restic to skip some startup operations and saves a bit of resources.

The `--parent` option is still available for `backup --stdin`, but is now ignored.

https://github.com/restic/restic/issues/3641
https://github.com/restic/restic/pull/3645