Easy to find commit on network graph

The commit, showing commit in the commit page or tree page, is marked and displayed in the center on network graph. (ref #2867)

### Screenshot

![easy-to-find-commit-on-network-graph](https://f.cloud.github.com/assets/72138/116996/3f69f7d6-6c15-11e2-9e93-4bb3e1da9343.png)