Update Porting/checkAUTHORS.pl to (once again) distinguish authors from committers...

Update Porting/checkAUTHORS.pl to (once again) distinguish authors from committers by processing git log --pretty=fuller, e.g.,

% git log --pretty=fuller perl-5.12.0..HEAD \
? | perl Porting/checkAUTHORS.pl --rank --percentage -

Without this, the --rank and --thanks-applied options were not working.

Also, the meaning of --thanks-applied has changed somewhat; it now only counts
commits where the committer is not the author.

[This commit is the delta between Craig's previous patch and his revised
patch]