SCM: Git clone performance

Via:
- http://stackoverflow.com/questions/1778088/how-to-clone-a-single-branch-in-git
- http://stackoverflow.com/questions/6941889/is-git-clone-depth-1-shallow-clone-more-useful-than-it-makes-out
- http://git-scm.com/docs/git-clone
Merge pull request #125 from xboston/patch-1

SCM: Git clone performance