When making a histogram of delta chain length in the pack, the program
collects number of objects whose delta depth exceeds the MAX_CHAIN limit
in histogram[0], and showed it as the number of items that exceeds the
limit correctly. HOWEVER, it also showed the same number labeled as
"chain length = 0".
In fact, we are not showing the number of objects whose chain length is
zero, i.e. the base objects. Correct this.