OSError will be raised in setup.py if "git" is not installed
OSError will be raised in setup.py if "git" is not installed

Summary: Closes https://github.com/caffe2/caffe2/pull/1771

Reviewed By: pietern

Differential Revision: D6777503

Pulled By: bddppq

fbshipit-source-id: 7ef66c1bdd6a1c410c3938566d5e8979e3bb5b12

Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,caffe2/caffe2