Enhancement: Self-heal missing file parts during backup of unchanged files

We've improved the resilience of restic to certain types of repository corruption.

For files that are unchanged since the parent snapshot, the backup command now
verifies that all parts of the files still exist in the repository. Parts that are
missing, e.g. from a damaged repository, are backed up again. This verification
was already run for files that were modified since the parent snapshot, but is
now also done for unchanged files.

Note that restic will not backup file parts that are referenced in the index but
where the actual data is not present on disk, as this situation can only be
detected by restic check. Please ensure that you run `restic check` regularly.

https://github.com/restic/restic/issues/2571
https://github.com/restic/restic/pull/2827
