Compute whether to publish or not based on output signature rather than commits

[Assuming we already have reproducible qualifiers...]

Currently, the CI jobs publish the output of a build if a change was detected in the Git repo since last publication.
However there are some case not correctly supported:

• Job configuration changed and caused changes in build output. In that case, we would like build output to be re-published, but it won't happen
• Job source changed but not build output (eg a change in a pom.xml or any non-included file), then we don't need to republish output whereas current mechanism would republish it.

instead, it would make sense to compare what's already published and what is to publish in order to decide or not whether this has to be published. Looking at signatures of update site zip would probably be enough.