Bugfix: Ignore unexpected files in the data/ folder

If the data folder of a repository contained files, this would prevent restic
from retrieving a list of file data files. This has been fixed. As a workaround
remove the files that are directly contained in the data folder (e.g.,
`.DS_Store` files).

https://github.com/restic/rest-server/issues/219
https://github.com/restic/rest-server/pull/221