ci: run Git's entire test suite

[dscho]

One particularly important part of Git for Windows' MSYS2 runtime is that it is used to run Git's tests, and regressions happened there: For example, the first iteration of MSYS2 runtime v3.5.5 caused plenty of hangs. This was realized unfortunately only after deploying the msys2-runtime Pacman package, and some painful vacation-time scrambling was required to revert to v3.5.4.

To verify that this does not happen anymore, let's reuse what setup-git-for-windows-sdk uses in Git's very own CI:

• determine the latest successful ci-artifacts workflow run in git-for-windows/git-sdk-64
• download its Git files and build artifacts
• download its minimal-sdk
• overwrite the MSYS2 runtime in the minimal-sdk
• run the test suite and the assorted validations just like the ci-artifacts workflow (from which these jobs are copied)

This obviously adds a hefty time penalty to every MSYS2 runtime PR in the git-for-windows org. Happily, these days we don't need many of those, and the balance between things like the v3.5.5 scramble and waiting a little longer for the CI to finish is clearly in favor of the latter.

[jeremyd2019]

The 'upstream' of these tests is git-for-windows/git? I ask because I think this could be a candidate for a callable/reusable workflow to avoid duplicating these jobs. I want to try that and possibly open some PRs to make it so if it works out.

[dscho]

The 'upstream' of these tests is git-for-windows/git?

Define "upstream" ;-)

This workflow downloads artifacts that are built by git-sdk-64's ci-artifacts workflow, which build's and tests git/git's master branch of the day.

I ask because I think this could be a candidate for a callable/reusable workflow to avoid duplicating these jobs.

I did try with Git for Windows' GitHub workflows to put logic into reusable workflows (or more like, composite Actions, because I find them easier to use and maintain), but I gave up. GitHub Actions' design is kind of recapitulating parts of the history of programming, where e.g. the value of inheritance and sub-classing was discovered and re-discovered, but since investment in GitHub Actions has all but stopped, it never got quite where I need it to be. Sure, I can emulate some reusable component design by moving logic into composite Actions with a gazillion inputs and then use if: conditions quite liberally, but it gets unreadable really quickly.

I want to try that and possibly open some PRs to make it so if it works out.

Sure, I won't stop you, and I will be delighted to see your beautiful solution!