Automatically merge `msys2-runtime` PRs when their corresponding `MSYS2-packages` PRs were deployed successfully

[dscho]

As of #87, /open pr supports opening MSYS2-packages PRs also for msys2-runtime PRs. This is very different from the way /open pr is used otherwise, namely in component-updates issues. As such, the, say, MSYS2-packages PR can be opened in a way that automatically closes such a corresponding component-updates issue, but the same is not true for the msys2-runtime PRs.

Let's close those msys2-runtime PRs by "pushing" the PR branch (which in reality will look more like updating the default branch using the PR's head.sha, like we already do to "merge" git-for-windows/git release PRs).

Alternatively, the msys2-runtime PR could be merged when the MSYS2-packages PR is merged, which is more in line with the manner component-update issues are automatically closed.