Only format git diff (similar to clang-format-diff)

Relevant yapf issue: https://github.com/google/yapf/issues/190

It would be really helpfull to only format code that was changed which also helps to minimize the diff.

`clang-format-diff` example:
```sh
git diff -U0 --no-color HEAD^ | clang-format-diff.py -p1 -i
svn diff --diff-cmd=diff -x-U0 | clang-format-diff.py -i
```