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 files in it were damaged and other
files were still fine.

Restic now has a `repair snapshots` command, which can salvage any non-damaged
files and parts of files in the snapshots by removing damaged directories and
missing file contents. Please note that the damaged data may still be lost
and 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