Bugfix: Hide password in REST backend repository URLs

When using a password in the REST backend repository URL,
the password could in some cases be included in the output
from restic, e.g. when initializing a repo or during an error.

The password is now replaced with "***" where applicable.

https://github.com/restic/restic/issues/2241
https://github.com/restic/restic/pull/2658
