borg extract from related repos

Just to keep a borg2 idea:

Assume we have multiple **related repos** that get backups of basically the same source data sets:
- REPO_FAST (e.g. it is local, same LAN or on a local disk even)
- REPO_SLOW (e.g. somewhere remote on an internet server, low throughput, high latency)

The backup archives in these repos could be:
- totally or partly identical (e.g. someone used `borg transfer`)
- all slightly different (someone did just independent backups to both repos)
- same, but differently pruned
- a mix of all of this

Assume we want to extract an archive from the slow repo (e.g. because it is somehow newer/better than the ones in the fast repo), we could implement code for this:
```
borg --repo REPO_SLOW extract --other-repo=REPO_FAST ARCHIVE_SLOW ...
```

borg would then **fetch chunks from the related fast repo if they exist there**, but the result would be the same as if it fetched everything from the slow repo.