When checking a checksum that refers to a part of an --inplace file that
has been overwritten w/o getting SUMFLG_SAME_OFFSET set, we remove the
checksum from the list.  This will speed up files that have a lot of
identical checksum blocks (e.g. sequences of zeros) that we can't use
due to them not getting marked as being the same.  Patch provided by
Michael Chapman.