title: How do I "mount" one github repository as a subfolder of another repository?
body: <p>I have a github repository named "foo" (or rather "myusername/foo"). Now suppose I want to create a repository named "bar", which uses the code in foo; I could just make a copy of the files, but I don't want to need to pull updates - I want to always see only whatever's in the "foo" repository right now; and not to have commits of "updates from foo". I believe I've noticed some repositories in which subfolders are actually separate repositories; and I know git supports sub-repositories or some such thing. So, can I perform such a "repository mount"? If so, how?</p>
  
  <p>Bonus points if I can have the "mount" at a certain version, or a certain branch, rather than the master</p>