Bugfix: Fix creation of oversized indexes by `repair index --read-all-packs`

Since restic 0.17.0, the new index created by `repair index --read-all-packs` was
written as a single large index. This significantly increases memory usage while
loading the index.

The index is now correctly split into multiple smaller indexes. `repair index` now
also automatically splits oversized indexes.

https://github.com/restic/restic/pull/5249