Fix up diffcore-rename scoring

Fix up diffcore-rename scoring

The "score" calculation for diffcore-rename was totally broken.

It scaled "score" as

	score = src_copied * MAX_SCORE / dst->size;

which means that you got a 100% similarity score even if src and dest were
different, if just every byte of dst was copied from src, even if source
was much larger than dst (eg we had copied 85% of the bytes, but _deleted_
the remaining 15%).

That's clearly bogus. We should do the score calculation relative not to
the destination size, but to the max size of the two.

This seems to fix it.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>