Enhancement: Add bitrot detection to `diff` command

The output of the `diff` command now includes the modifier `?` for files to
indicate bitrot in backed up files. The `?` will appear whenever there is a
difference in content while the metadata is exactly the same.

Since files with unchanged metadata are normally not read again when creating
a backup, the detection is only effective when the right-hand side of the diff
has been created with `backup --force`.

https://github.com/restic/restic/issues/805
https://github.com/restic/restic/pull/4526
