Add a summary about how many files are reproducible in an archive

There might be cases where we are interested in having a comparing on "how much" two sets of files are unreproducible, and for example decide on which variant could be more easy to work on.

One simple enough way to go about it is reporting a percentage of how many files are not reproducible between the two tested archives. That could easily be part of the report in an unreproducible output.