bugfixes:

fixed incorrect preliminary prune that was using a stale iterator over
intersection set of lines; now using new HashSet to contain set intersection

fixed deterministic pairing that was using an iterator over concurrent hashset
whose ordering was not guaranteed; now using concurrent skiplist set that /does/
have guaranteed ordering
bugfix:

corrected the way total files are counted

corrected the way file diffs are counted; i.e., a file_comparison_result
represents a file diff if it has the following: unmatched lines, reportable
pairs, and/or non-identical report headers