When the graph gets too wide (as defined by the maxwidth variable,
which can be set in ~/.gitk), we can now terminate graph lines with
an arrow pointing downwards, and reintroduce them later with an
arrow pointing upwards when we need them.  This makes the graph much
less cluttered on large repositories such as the linux kernel.

Unfortunately this has made it slower; it takes about 10 seconds
user time on the linux-2.6 repository on my machine now, compared
to 6 seconds before.  I'll have to work on optimizing that.  Also
on the todo list are making the arrow heads active (so if you click
on them you jump to the other end) and improving the placement of
the null entry.