borg2: avoid doing an unnecessary rebuild of the index

Note: updated for situation WITH packs.

For high-latency stores (`sftp:` and a lot of what's available via `rclone:`) a full index rebuild needs to list all objects in all packs in the store and there are potentially a lot of objects. This is slow as it needs to scan over each pack file - only borg check --repair-index shall do that.

Most commands will use the existing chunks index. Most commands only add objects, so they will just write an additional partial index to index/sha256(partial_index) with just the new index entries.

For borg create:
- it's not a big problem if the index does not have all objects that exist in the repo. If that happens, borg create will just store something to the repo that's already there.
- it would be a severe problem though if the index would falsely say "we have that object" and borg would not store it to the repo. the archive would then reference a non-existing object.
- similarly it is fatal if the index points to a wrong pack_id