diffcore-break/diffcore-rename: integer overflow.

diffcore-break/diffcore-rename: integer overflow.

While reviewing the end user tutorial rewrite by J. Bruce
Fields, I noticed that "git-diff-tree -B -C" did not correctly
break the total rewrite of Documentation/tutorial.txt.  It turns
out that we had integer overflow during the break score
computations.

Cop out by using floating point.  This is not a kernel.

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