Convert GitLab patches to simple diffs

GitLab unlike GitHub keeps the footer from git-format-patch(1) which
frequently changes on Git version upgrades. So, switch to git-diff(1)
which lacks header/footer.

Approved by: x11 (zeising via Gitter)
MFH: 2020Q2 (avoid bustage over time due to upstream infra upgrades)
Differential Revision: https://reviews.freebsd.org/D24810