Allow --skip-hash and --skip-compare to be specified together

Since --skip-hash is now a threshold parameter which applies to some
files and not others, it makes sense to allow --skip-compare to be
specified at the same time.

--skip-compare is respected for files smaller than the --skip-hash
threshold, since such files will always be hashed.  --skip-compare is
ignored for files larger than or equal to the --skip-hash threshold
since such files will never be hashed.