Bugfix: Limit number of key files tested while opening a repository

Previously, restic tested the password against every key in the repository, when
there are more and more keys in the repository, opening the repository becomes
slower and slower.

Now restic tests password against up to 20 key file in the repository. Alternatively,
you can use `--key-hint=<Key ID>` to specify the key file to be used.

https://github.com/restic/restic/pull/3776
