git-pickaxe: introduce heuristics to avoid "trivial" chunks

git-pickaxe: introduce heuristics to avoid "trivial" chunks

This adds scoring logic to blame_entry to prevent blames on very
trivial chunks (e.g. lots of empty lines, indent followed by a
closing brace) from being passed down to unrelated lines in the
parent.

The current heuristics are quite simple and may need to be
tweaked later, but we need to start somewhere.

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