Enhancement: Ignore additional/unknown files in repository

If a restic repository had additional files in it (not created by restic),
commands like `find` and `restore` could become confused and fail with an
`multiple IDs with prefix "12345678" found` error. These commands now
ignore such additional files.

https://github.com/restic/restic/pull/3943
https://forum.restic.net/t/which-protocol-should-i-choose-for-remote-linux-backups/5446/17
