[#5738] Add instrumentation to Commit._diffs_copied

Signed-off-by: Cory Johns <johnsca@geek.net>
[#5738] Use real_quick_ratio instead of ratio for performance

Signed-off-by: Cory Johns <johnsca@geek.net>
[#5738] only instrument SequenceMatcher ratio methods, not internal methods
[#5738] Use quick_ratio instead of real_quick_ratio to reduce false-positive on _diffs_copied

Signed-off-by: Cory Johns <johnsca@geek.net>
[#5235] ticket:255 set last_post_date to anonymous' thread