git submodules hook/support ala Heroku

_From @azurewraith on June 2, 2014 20:18_

I attempted to reproduce some Heroku'esque workflow involving a vendored Redmine plugin and found this:

https://devcenter.heroku.com/articles/git-submodules

90% sure it probably wouldn't have worked for me because git submodules are a PITA but it seems like Heroku handles this in their post-receive hook before delegating to the buildpack...

_Copied from original issue: deis/deis#1094_