borg check: doing a repair using redundant related repos

borg2: we can now have multiple "related" repos based on the same key material.

this includes the key for the id hash / MAC and also the chunker secret, thus chunking will happen in the same way and also the chunkid computation will happen in the same way.

so, if you make 2 individual backups (at approximately the same time) into 2 such repos, there will be a huge (although not necessarily 100%) overlap of chunkids with the respective content.

if one would use `borg transfer` to copy an archive to another related repo, there would be even a 100% overlap of chunkids for this archive.

even if both repos lose some chunks, there would be a good chance that each of these chunks can still be found in one of them.

guess the only missing piece is some code helping with such repairs.

could be something like: `borg --repo=REPO_TO_REPAIR check --other-repo=REPO_TO_READ_FROM --repair`