git push github master

git svn fetch svn-core
git checkout svn-core
git merge remotes/svn-core
git checkout -f master # force because of mythos subtree "overwrite"
git subtree merge -P core --squash -m "merge upstream svn to core subtree" svn-core
git checkout git-core # warning: mythos dir not empty
git merge svn-core    # fast forward
git push github master
git push github git-core:svn-core
git checkout -f master

git subtree split -P core -b svn-core
git checkout svn-core
git svn dcommit
git push github git-core:svn-core
# NB! merge following dcommit
git checkout master
git subtree merge -P core --squash -m "merge dcommit" svn-core