Bugfix: `repair packs` correctly handles pack files missing from the index

The `repair packs` command was unable to salvage blobs from a pack file if
the pack file was not contained in the index or the index entry was incomplete.
The command now uses both the information contained in the index and the
pack file header.

https://github.com/restic/restic/issues/21820
https://github.com/restic/restic/pull/21827
