repozo: allow --verify to check backup against source Data.fs

When using quick backups (-BQ), repozo only checksums the byte range of the last increment against the source file. If earlier backup files are corrupted, quick backups will not detect it and will keep appending increments on top of a potentially inconsistent base.

The --verify (-V) mode now accepts --file (-f) to verify the backup against the source Data.fs, by comparing md5 checksums of the concatenated backup data with the corresponding bytes of the source. With --quick, only sizes are compared.