Enhancement: Add `repair index` and `repair snapshots` commands

The `rebuild-index` command has been renamed to `repair index`. The old name
will still work, but is deprecated.

When a snapshot was damaged, the only option up to now was to completely forget
the snapshot, even if only some unimportant file was damaged.

We've added a `repair snapshots` command, which can repair snapshots by removing
damaged directories and missing files contents. Note that using this command
can lead to data loss! Please see the "Troubleshooting" section in the documentation
for more details.

https://github.com/restic/restic/issues/1759
https://github.com/restic/restic/issues/1714
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