I assume the original code where they started from is available and then the final released code. You can import both in git with the second over the first and you then can easily see the OS4 changes. For a programmer worth it's name and familiar with the Amiga(OS) API that should already tell a lot even without documentation. You should even be able to do this in subversion but git will allow you to split up the big patch in smaller patches much easier. The latter does need quite deep knowledge of git features as history rewriting etc. though.
Well you need to find the extract revision their code is based from (perhaps there is some information in the repository to point to this), you could then reconstruct at least the firefox history part, and then apply the amigaos changes on top. But my basic point was that it looks as though they didn't go our their way to make this easy - I'm sure they used source control themselves (who would work on such a big source without it ?), so where is it. It would be very useful to see the changelog of the AmigaOS parts.
Thanks for the git lesson ;-)
