Copy commits from the main repository to the ironpython3 repository:

pushd ironlanguages-main
git checkout $branch
git format-patch -o $branch-patches $(git merge-base -a master $branch)
git checkout -
popd
pushd ironpython3
git checkout -b $branch
git am --include="*/Languages/IronPython/*" -p 3 --directory=Src ../ironlanguages-main/$branch-patches/*
# fix merge conflicts as needed
popd
rm -r ironlanguages-main/$branch-patches

This can be tweaked to update a branch by not using -b on checkout and changing the use of merge-base to a hardcoded revision. Commits that update files outside of Languages/IronPython may need extra include lines, although the directory structure is similar enough (for this reason) that it should just work. Worst case, edit the patch files by hand before using git am (the hashes will change anyway).

Something similar can be done for the Dlr as well by tweaking the paths.

As above, but change the patching line to:

git am --include="*/Src/*" -p 3 --directory=Languages/IronPython ../ironpython3/$branch-patches/*