Compress the graph horizontally if it gets too wide.

Compress the graph horizontally if it gets too wide.

If the graph gets to use more than a certain percentage (default 50%)
of the width of the top-left pane, we now reduce the amount of space
allowed for each graph line.  This means it doesn't look quite as
nice but you can still see the headline for the commit.  (Currently
the only way to customize the percentage is to edit your ~/.gitk
file manually.)