Enhancement: Add new command repair

We've added a new command repair which allows to repair snapshots even if needed
parts of it are not accessable in the repository. Note that using this command
can lead to data loss!

Some corrupted repositories were reported in several issues and so far restic
lacked a possibility to accept data loss but clean those up such that the
repository returns to a sane state. This possibility was now added.

https://github.com/restic/restic/issues/1759
https://github.com/restic/restic/issues/1798
https://github.com/restic/restic/issues/2334
https://github.com/restic/restic/pull/2876
https://forum.restic.net/t/corrupted-repo-how-to-repair/799
https://forum.restic.net/t/recovery-options-for-damaged-repositories/1571