Hello Bruno,
btw, why is svn still needed? I see that it is only used for ExternalLibraries which are anyway in a git repository (is it because of the particular directory structure of that repository?).
Not so much because of the directory structure but because of the content. We use svn for the ExternalLibraries thorns, which contain a tarball of the library (in case no copy is found on the system).
Since git downloads the full version history (by default, and shallow, --depth 1 checkouts do not work for us when using branches) we did not want to use git for those (since eg the MPI ExternalLibrary repo is ~172MB of "old" commits and ~10MB of checked out files. svn can download a single revision more easily.
Yours, Roland