diff --stat: ensure at least one '-' for deletions, and one '+' for additions

diff --stat: ensure at least one '-' for deletions, and one '+' for additions

The number of '-' and '+' is still linear. The idea is that
scaled-length := floor(a * length + b) with the following constraints: if
length == 1, scaled-length == 1, and the combined length of plusses
and minusses should not be larger than the width by a small margin. Thus,

	a + b == 1

and
	a * max_plusses + b + a * max_minusses + b = width + 1

The solution is

	a * x + b = ((width - 1) * (x - 1) + max_change - 1)
		 / (max_change - 1)

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>