Add check to ensure all patches are contributions

This essentially just checks that commit messages do not contain the
string 'not a contribution' as such patches cannot be accepted into
Chromium OS.

BUG=None
TEST=This commit message fails to upload.

Change-Id: Ica5e48dde7320a5a2e7c0bd234fbc98f57e90d49
Reviewed-on: https://chromium-review.googlesource.com/321798
Commit-Ready: Bernie Thompson <bhthompson@chromium.org>
Tested-by: Bernie Thompson <bhthompson@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed