Bugfix: Correct `--no-lock` handling of `ls` and `tag` commands

The `ls` command never locked the repository. This has now been fixed, with the
old behavior still being supported using `ls --no-lock`. The latter invocation
also works with older restic versions.

The `tag` command erroneously accepted the `--no-lock` command. This command
now always requires an exclusive lock.

https://github.com/restic/restic/pull/4709
