Fix detection of path conflicts.

- os.path.commonprefix does not work as expected to return
   path component prefixes, but rather works on character
   by character basis.

A real example of a .rosinstall file (generated by rosinstall_generator) for which init failes without this patch:

```
- tar:
    local-name: libuvc
    uri: https://github.com/ktossell/libuvc-release/archive/release/hydro/libuvc/0.0.4-1.tar.gz
    version: libuvc-release-release-hydro-libuvc-0.0.4-1
- tar:
    local-name: libuvc_ros/libuvc_camera
    uri: https://github.com/ktossell/libuvc_ros-release/archive/release/hydro/libuvc_camera/0.0.7-0.tar.gz
    version: libuvc_ros-release-release-hydro-libuvc_camera-0.0.7-0
```