https://github.com/dflydev/git-subsplit

laravel 에서도 사용하고 있는 방법.

git subsplit init git@github.com:laravel/framework.git

git subsplit publish --heads="master 5.2 5.1" --no-tags src/Illuminate/Auth:git@github.com:illuminate/auth.git
git subsplit publish --heads="master 5.2 5.1" --no-tags src/Illuminate/Broadcasting:git@github.com:illuminate/broadcasting.git
git subsplit publish --heads="master 5.2 5.1" --no-tags src/Illuminate/Bus:git@github.com:illuminate/bus.git
git subsplit publish --heads="master 5.2 5.1" --no-tags src/Illuminate/Cache:git@github.com:illuminate/cache.git

https://github.com/laravel/framework/blob/5.3/build/illuminate-split.sh

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