"3. The GPL requires RedHat to share the source of the final product that is distributed with those it distributes to. Those receiving that source can use common GNU tools for building patches themselves, though this will not help them understand much about why the differences are there."
Minor caveat: This is true IFF the files have diligently been marked up, as required by the license, with the nature of the changes made to them in each successive change and all of those markings (if there are multiple changes in a file) have been preserved. If, as is often the case, that information is only in the changelog, then you can't reconstruct the successive patches, but only a single patch that applies the net result of the whole sequence of changes to a particular file.