frail url mutation for tar ball snippets

[wjwwood]

I just ran into this problem where the rosinstall snippets generated for some packages in indigo are invalid. For example, eigen_stl_containers's snippet:

% rosinstall_generator eigen_stl_containers --rosdistro indigo --tar
- tar:
    local-name: eigen_stl_containers
    uri: https://github.com/ros-gbp/eigen_stl_containers-release
    version: eigen_stl_containers-rel-release-indigo-eigen_stl_containers-0.1.4-0

You can see here that the uri is not pointing to a tarball. By comparison a valid snippet is:

% rosinstall_generator eigen_stl_containers --tar
Using ROS_DISTRO: hydro
- tar:
    local-name: eigen_stl_containers
    uri: https://github.com/ros-gbp/eigen_stl_containers-release/archive/release/hydro/eigen_stl_containers/0.1.4-0.tar.gz
    version: eigen_stl_containers-release-release-hydro-eigen_stl_containers-0.1.4-0

I have determined that this is because the entry in the distribution.yaml file for Indigo is missing the .git extension:

  eigen_stl_containers:
    release:
      tags:
        release: release/indigo/{package}/{version}
      url: https://github.com/ros-gbp/eigen_stl_containers-release
      version: 0.1.4-0

I have opened a pull request to fix those missing extensions and add a check for rosdistro CI to check for them here:

https://github.com/ros/rosdistro/pull/3204/files

Ultimately, even if we fix the missing extensions in the rosdistro file, this line of code:

https://github.com/ros-infrastructure/rosinstall_generator/blob/master/src/rosinstall_generator/distro.py#L135

Is pretty fragile. We should at least assert that it ends with .git and fail because we cannot convert it otherwise.