gitlab-ci: exercise Git on Windows

gitlab-ci: exercise Git on Windows

Add jobs that exercise Git on Windows. Unfortunately, building and
especially testing Git on Windows is inherently slower compared to other
Unix-like systems, mostly because spawning processes is way slower. We
thus use the same layout as we use in GitHub Actions, where we have one
build job, and then pass on the resulting build artifacts to ten test
jobs that split up the work across each other.

Unfortunately, the GitLab runners for Windows machines are embarassingly
slow by themselves. So while this strategy leads to around 20 minutes of
build time in GitHub Actions, the same pipeline takes around an hour in
GitLab CI. Still, having late coverage is certainly better than having
none at all.

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