Add a threshold to skip-hash

Hashing usually performs well except in special cases where many
large files have the same size but very different contents near
the beginning of the file.  In these cases, it is usually faster
to execute the O(N^2) comparisons between all the files, thereby
avoiding reading most of their data.

In cases where there are many small files, the opposite setting of
the skip-hash option is usually better, so skip-hash is now a
threshold parameter with a default of 1MB.

Unfortunately it is not trivial to detect this condition without
doing sufficient work to negate the benefit; hence, we require
the operator to specify a preference.