A manual cross-repository rebase

xrebase

# If you haven't already, clone the new repo and `cd` into its directory.
git clone git@github.com:username/new-repo.git
cd new-repo

# Add the old repo as a remote and fetch from it.
# If you're trying to do this from an unpushed branch,
# git remote add old-repo ~/dev/old-repo.git
git remote add old-repo git@github.com:username/old-repo.git
git fetch old-repo

# Create local branches that track the old-repo's develop and feature.
git checkout -b old-develop --track old-repo/develop
git checkout -b feature --track old-repo/feature

# Rebase from old-develop..feature onto develop.
# That is, starting at develop, recommit the commits that comprise feature.
git rebase -s subtree --onto develop old-develop feature

# Push the newly-rebased feature to origin.
git push --set-upstream origin feature