Enhancement: New option --repository-file

We've added a new command-line option --repository-file as an alternative
to -r. This allows to read the repository URL from a file in order to
prevent certain types of information leaks, especially for URLs containing
credentials.

https://github.com/restic/restic/issues/1458
https://github.com/restic/restic/issues/2900
https://github.com/restic/restic/pull/2910
