README: add GitLab CI badge to make it more discoverable

README: add GitLab CI badge to make it more discoverable

The Git project uses CI systems from both GitHub and GitLab. While both
of these systems are extensively used in day-to-day work, we only have a
link to the GitHub Workflows in our README, which makes the GitLab CI
hard to discover.

Improve the situation by adding a second badge for GitLab CI to our
README.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>