merge a subdirectory from another repo while preserving history

subtree-merge.sh

# credit: http://jrsmith3.github.io/merging-a-subdirectory-from-another-repo-via-git-subtree.html
# Clone the target repo
git clone git@github.com:jclouds/jclouds.git
cd jclouds

# Add the source repository as a remote, and perform the initial fetch.
git remote add -f sourcerepo git@github.com:jclouds/jclouds-labs-openstack.git

# Create a branch based on the source repositories' branch that contains the state you want to copy.
git checkout -b staging-branch sourcerepo/master

# Here's where the two approaches diverge.
# Create a synthetic branch using the commits in `/openstack-glance/` from `sourcerepo`
git subtree split -P openstack-glance -b openstack-glance

# Checkout `master` and add the new `openstack-glance` branch into `/apis/openstack-glance/`. At this point, the desired result will have been achieved.
git checkout master
git subtree add -P apis/openstack-glance openstack-glance

# Clean up by removing the commits from the `openstack-glance` branch and the `sourcerepo` remote.
git branch -D openstack-glance staging-branch
git remote rm sourcerepo