Extend `open-pr` to open MSYS2-packages PRs corresponding to msys2-runtime PRs

[dscho]

Today, I faced the relatively simple task of opening an MSYS2-packages companion to git-for-windows/msys2-runtime#52. This task was complicated by me being essentially away from a real computer all day. I did open git-for-windows/MSYS2-packages#107 eventually, but the entire point of git-for-windows-automation is to allow performing such repetitive and straight-forward tasks even via cell phone.

Let's teach the open-pr GitHub workflow the trick to allow /open pr to be called in (merged!) git-for-windows/msys2-runtime PRs.

The steps are straight-forward:

• first make sure that the git-for-windows/msys2-runtime PR is merged (otherwise, the update-patches.sh step below will fail to do what is intended)
• clone MSYS2-packages
• create a new branch with the same name as the git-for-windows/msys2-runtime PR, but prefixed with msys2-runtime, and with origin/main as upstream
• in the msys2-runtime/ subdirectory:
  • initialize a partial clone of git-for-windows/msys2-runtime in src/msys2-runtime
  • call the ./update-patches.sh script
  • commit the result with a commit message that is crafted from the git-for-windows/msys2-runtime PR's description
• push the branch
• open a PR in git-for-windows/MSYS2-packages (transmogrifying the description of the git-for-windows/msys2-runtime PR)