backup: Clean target paths before finding parent
backup: Clean target paths before finding parent

This resolves an issue described in the forum where restic could not
find a parent snapshot if the target path ends with a slash:
https://forum.restic.net/t/new-archiver-code-please-test/623/23

kurin/restic,middelink/restic,restic/restic,kurin/restic,middelink/restic,restic/restic