Remove git refs used internally by GitLab from network graph

GitLab 8.0 removed satellites and started using git refs that began with `refs/merge-requests/XXX` and `refs/tmp/XXX`, which showed up in an ugly way in the network graph:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/150eb7ae5f07f0d4793589b5eac462db/image.png)


Closes #2702

See merge request !1410