[ros-release] Ease comparing versions of packages between ROS distros

[Dirk Thomas]

For maintainers a common set of questions regarding multiple ROS distributions are:

* after releasing patch releases into Hydro, did I already released the same version into Groovy?
* which versions of a repo are released in the current ROS distros?

To answer those faster then looking up the relevant information manually I generated some additional "compare version" pages.
They can be found in the same location as the Debian package status pages: http://www.ros.org/debbuild/

Currently the following variants exist:

* Groovy vs. Hydro: http://www.ros.org/debbuild/compare_groovy_hydro.html
* Hydro vs. Indigo: http://www.ros.org/debbuild/compare_hydro_indigo.html
* all current distros (G, H and I): http://www.ros.org/debbuild/compare_groovy_hydro_indigo.html

Some predefined filters exist which allow to narrow down the list of repos based on the following criteria:

* different patch version:
   The packages from a repo in two ROS distros have the same major and minor version but a different patch version.
   Usually that implies that you should update the distro which lacks a bit behind.
   Temporary that might be the case when you first release a new version into one distro and after some soak time release it into the other distros.

* downgrade:
   The packages from a repo in a newer ROS distro have a lower major or minor version number.
   The maintainer should reconsider the versioning of the repo and rerelease the packages from the repo with proper version numbers.

* same version from different branches:
   The packages from a repo have the same major and minor version (potentially a different patch version) but are released from different branches.
   That usually indicates a problem since it is likely that there could be or will be two packages with the same version but different code (since both branches might need to perform patch releases 
over time).
   Commonly the maintainer would increase the minor version when branching for the next ROS distro to avoid this kind of situation.

I hope you will find these pages useful when maintaining ROS packages.

Cheers,
- Dirk