Fix style of network graph.

1. Incerease width of panel with month names(refs #3258)
2. Use arrowed line only if it connects to not first parent
 * It is easy to see which branch is merged.

### Before

![allowed_line_if_not_first_parent_before](https://f.cloud.github.com/assets/72138/289738/09c2c610-92c3-11e2-8755-ef52f179db3c.png)

### After

![allowed_line_if_not_first_parent_after](https://f.cloud.github.com/assets/72138/289740/1310485a-92c3-11e2-82ec-01b9df6ebc9e.png)