A script that automatically reformat affected lines

Summary:
Added a script that reformat only the affected lines in a given diff.

I planned to make that file as pre-commit hook but looks it's a little bit more difficult than I thought. Since I don't want to spend too much time on this task right now, I eventually added a "make command" to achieve this with a few additional key strokes.

Also make the clang-format solely inherited from Google's style -- there are still debates on some of the style issues, but we can address them later once we reach a consensus.

Test Plan: Did some ugly format change and ran "make format", all affected lines are formatted as expected.

Reviewers: igor, sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15147