clang-format: add config file and gitlab-ci stage

Add a copy the clang-format config from virgl with a few minor tweaks.

There are a few trivial changes, but no something we should bother with updating in one go. Instead add a gitlab-ci stage to handle that.

Some of the branch (merge-base) logic was shamelessly copied from ci-fairy - props to the original authors of that tool.

Note: the clang-format stage does not block the build, plus there's a dummy commit on top (for illustration) that I will remove before merging