Bugfix: List all files stored in Google Cloud Storage

For large backups stored in Google Cloud Storage, the `prune` command fails
because listing only returns the first 1000 files. This has been corrected, no
data is lost in the process. In addition, a plausibility check was added to
`prune`.

https://github.com/restic/restic/issues/1246
https://github.com/restic/restic/pull/1247