Long branch names overlap commits

[aroben]

When a branch name is quite long, it ends up overlapping some commits:

         o--o--o <-- new head of origin
        /
 o--o--O--o--o--o <-- old head of origin

[aroben]

This is happening because we aren't calculating positions in dot coordinates correctly. We record the X coordinate of the left side of the branch label, but dot wants us to give it the X coordinate of the center of the branch label.