Fix automatic forward port in ROOT6

I think this is the correct one. In order to merge it I did:

```
git checkout cms-sw/CMSSW_7_3_ROOT6_X
git merge -X ours cms-sw/CMSSW_7_3_X
git revert 3a828c390bc11cfa348a6baf77b9849c3d4ce250
```

so that:

- All the changes in ROOT6 win over CMSSW_7_3_X.
- The commit which @wmtan does not want is discarded.

Let's see if this works...