git-pickaxe: improve "best match" heuristics

git-pickaxe: improve "best match" heuristics

Instead of comparing number of lines matched, look at the
matched characters and count alnums, so that we do not pass
blame on not-so-interesting lines, such as an empty line and
a line that is indentation followed by a closing brace.

Add an option --score-debug to show the score of each
blame_entry while we cook this further on the "next" branch.

Signed-off-by: Junio C Hamano <junkio@cox.net>