Enhancement: Improve features of the `repair packs` command

The `repair packs` command has been improved to also be able to process
truncated pack files. The `check` and `check --read-data` command will provide
instructions on using the command if necessary to repair a repository. See the
guide at https://restic.readthedocs.io/en/stable/077_troubleshooting.html for
further instructions.

https://github.com/restic/restic/issues/828
https://github.com/restic/restic/pull/4644
https://github.com/restic/restic/pull/4882
