How to migrate part of a Drupal project to a new project repository

This is a writeup of a problem I encountered when working on the CollectiveAccess Drupal module: I had been maintaining a submodule called CollectiveAccess Feeds within the main Drupal CollectiveAccess project, but I found out that it would be more practical to have this as a separate module, in order to move it faster then the CollectiveAccess module itself, which is already more stabilized. Below are the steps I took to migrate this module to a new Drupal project. It might not be the most efficient or perfect solution, but it worked for me. If you know of better ways to achieve this, feel free to leave a comment.

Structural situation

To make my explanation more clear, here is the start situation:

* collectiveaccess
    |_ collectiveaccess.info
    |_ collectiveaccess.module
    |_ ... (more collectiveaccess module files)
    |_ modules
       |_ collectiveaccess_feeds
          |_collectiveaccess_feeds.info
          |_collectiveaccess_feeds.module

Here is what i wanted it to be:

* collectiveaccess
    |_ collectiveaccess.info
    |_ collectiveaccess.module
    |_ ... (more collectiveaccess module files)

* collectiveaccess_feeds
    |_collectiveaccess_feeds.info
    |_collectiveaccess_feeds.module

The following commands assume you have git maintainer access for the original Drupal project, and have created (or maintainership over) a second Drupal project, specifically for the spin off module.

Steps to achieve this:

Note: obviously you would have to change the project names for your own scenario!

1. Create a working directory:

   mkdir ~/migration
   cd migration

   Clone the current project:

2. git clone --branch 7.x-1.x \
   [drupal.org git username]@git.drupal.org:project/collectiveaccess.git

3. Create a separate project for the spin off module:

   git clone --no-hardlinks \
   collectiveaccess collectiveaccess_feeds

4. Filter the spin off module repository so only this module remains:

   cd collectiveaccess_feeds
   git filter-branch --subdirectory-filter \
   modules/collectiveaccess_feeds HEAD -- --all
   git reset --hard
   git gc --aggressive
   git prune

5. Still in the directory of your spin off project, we link the project to drupal.org:

   git remote rm origin
   git remote add origin \
   [drupal.org git username]@git.drupal.org:project/collectiveaccess_feeds.git
   git push origin 7.x-1.x

6. This step only applies if you have multiple branches:
   remove the spin off module directory

   cd ~/migration
   rm -rf collectiveaccess_feeds

   Now prepare yourself to repeat these steps for the 6.x-1.x branch (or another)

   cd collectiveaccess
   git checkout 6.x-1.x
   cd ~/migration

   Now repeat steps 3 to 5 for this particular branch. Repeat again for each branch you might have.
    
7. Clean up original repository
   remove all git clones in your migration directory & do a new clone:

   rm -rf ~/migration/*
   git clone --branch 7.x-1.x \
   [drupal.org git username]@git.drupal.org:project/collectiveaccess.git
   cd collectiveaccess

8. Remove the spin off code from the original project

   git rm -rf modules/collectiveaccess_feeds
   git add -A
   git commit -m "remove spin off code"
   git push origin

   Repeat this step for your other branches. Switch to them via the following command:

   git checkout 6.x-1.x

References used: http://stackoverflow.com/questions/359424/detach-subdirectory-into-separate-git-repository