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Some posts comparing different tools:
- http://blogs.atlassian.com/2013/05/alternatives-to-git-submodule-git-subtree/
- https://codingkilledthecat.wordpress.com/2012/04/28/why-your-company-shouldnt-use-git-submodules/
Pros and Cons
[TODO]After playing around with some and reading about pros and cons, it seems that Google repo fits best for Sling.