gitk: New algorithm for drawing the graph lines

This only draws as much of the graph lines as is visible.  This can
happen by adding coordinates on to an existing graph line or by
creating a new line.  This means that we only need to have laid out
and optimized as much of the graph as is actually visible in order to
draw it, including the lines (previously we didn't draw a graph
line until we had laid out and optimized to the end of a segment of
the line, i.e. down to a down-arrow or to the row where the line's
commit is displayed).  This also lets us get rid of the linesegends
list, and gives us an easy workaround for the X server bug that
causes long lines to be misdrawn.  This also gets rid of the use
of rowoffsets in drawlineseg et al.

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