Bugfix: Make `dump` interpret `--host` and `--path` correctly

A regression in restic 0.15.0 caused `dump` to confuse its `--host=<host>` and
`--path=<path>` options: it looked for snapshots with paths called `<host>`
from hosts called `<path>`. It now treats the options as intended.

https://github.com/restic/restic/issues/4211
https://github.com/restic/restic/pull/4212
