WIP check-style: Support incremental adoption of clang-format

Currently the use of clang-format is not mandatory. This patch rewrites
check-style to allow for incremental adoption of clang-format by making
it only mandatory for new files and files which were already formatted.

Features of the new check-style:
- Generates a patch for staged files that were not formatted before
- Automatically formats staged files that were formatted before (optional)
- Provides a mechanism to exclude files from clang-formatting
- Can format staged files only without touching the working tree
- Works with partially staged files
- Only requires clang-format when staged files were formatted before

Change-Id: Ib583503d268e002882af03c41b150f7c0fe17796
Signed-off-by: Lennart Eichhorn <lennarteichhorn@googlemail.com>
---
M Makefile.inc
M util/gitconfig/pre-commit
A util/lint/.enforce-style-ignore
D util/lint/check-style
A util/lint/enforce-style
5 files changed, 10,520 insertions(+), 148 deletions(-)