Automatically generate version upgrade archives from git

[fiammybe]

I am currently preparing the 1.4.4 release, and I remember from long ago that we also published upgrade archives. I also remember that it was a manual chore, so that's one of the reasons we left that on the side since then (in combination with the fact that nowadays, downloading a 10MB archive is not such a burden on your internet line anymore when compared to the dial-up era :-) )

However, it can be very interesting to see which files have been modified between branches, so I had a look around, and this looks promising : https://stackoverflow.com/questions/1678032/exporting-archiving-changed-files-only-in-git

It will be problematic if you deleted files between your branches, but it's a good start I think. If this exists as a github action, it might be interesting to see it work for ImpressCMS.