Bugfix: Correctly handle 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 information from both the index and the pack file header.

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