prune: Fix newly created index
prune: Fix newly created index

This fixes a bug introduced in c79fb6fcdd254382705753957bd299472696a2c6
where the index file after a prune contains pack files that do not exist
any more. Restic will detect this during backup and abort with an error
message until the user runs `prune` or `rebuild-index` again.

jayme-github/restic,jayme-github/restic,ar-jan/restic,kurin/restic,restic/restic,fawick/restic,ar-jan/restic,kurin/restic,fawick/restic,restic/restic,middelink/restic,middelink/restic