Change: Backups from stdin no longer have a parent

Snapshots made with `restic backup --stdin` no longer have a parent snapshot.
Since parent snapshots are only used to skip files in the scanning phase
based on filename and timestamps, the parent snapshot of a `--stdin` snapshot
was meaningless to begin with. Not setting a parent allows `restic backup`
to skip some startup operations.

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

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