Make git-clang-format python 3 compatible

Summary: This patch attempts to make `git-clang-format` both python2 and python3 compatible. Currently it only works in python2.

Reviewers: modocache, compnerd, djasper, jbcoe, srhines, ddunbar

Reviewed By: jbcoe

Subscribers: kimgr, mgorny, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D30773

llvm-svn: 303871