Add clang-tidy-diff.py script to run clang-tidy and display warnings on changed lines only.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4288

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@211698 91177308-0d34-0410-b5e6-96231b3b80d8