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. This can be fixed by running `repair packs`,
and 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 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
