gitk: Make downward-pointing arrows end in vertical line segment

It seems Tk 8.4 can't draw arrows on diagonal line segments.  This
adds code to the optimizer to make the last bit of a line go vertically
before being terminated with an arrow pointing downwards, so that
it will be drawn correctly by Tk 8.4.

Signed-off-by: Paul Mackerras <paulus@samba.org>