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

Previously, restic tested the password against every key in the repository
when opening a repository. The more keys there were in the repository, the
slower this operation became.

Restic now tests the password against up to 20 key files in the repository.
Alternatively, you can use the `--key-hint=<key ID>` option to specify a
specific key file to use instead.

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