Bugfix: Correct handling of duplicate index entries

Before restic 0.10.0, a bug could in very rare cases split information
about a pack file across multiple index files. Since restic 0.17.0, any
operation that rewrites the index (like `prune` or `repair packs`)
could lose part of that information, resulting in errors in later
`check` or `prune` runs. Those can be fixed by running `repair packs`.
Note that only repositories using repository format version 1 might be affected.

Split pack index entries are no longer lost during index rewrites. The
`check` command now reports these cases as errors that can instead be fixed using
the `repair packs` command. On older restic versions, running `repair index`
twice also fixes the problem.

https://github.com/restic/restic/issues/21820
https://github.com/restic/restic/pull/21828
