Lock the .clang-format style globally at the style used by clang-format 5.0,
so that we do not get issues with future versions of clang-format having
different defaults (due to new features), leading to style flip-flopping
as different people check in code. Reformat all code marked as such under
.gitattributes with clang-format 5.0.

Also add an example .gitconfig containing a clean filter.

Change-Id: Ie8e5660b07612155606555717ce583c76be56589