gitk: Better workaround for arrows on diagonal line segments

gitk: Better workaround for arrows on diagonal line segments

Instead of adding extra padding to create a vertical line segment at
the lower end of a line that has an arrow, this now just draws a very
short vertical line segment at the lower end.  This alternative
workaround for the Tk8.4 behaviour (not drawing arrows on diagonal
line segments) doesn't have the problem of making the graph very wide
when people do a lot of merges in a row (hi Junio :).

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