/git-artifacts: verify that the git-sdk-* commits are newer than the corresponding Pacman indices

[dscho]

Every once in a while we have to /deploy packages on the same day we release a new Git for Windows version. For procedural reasons, this requires the sync workflows of the git-sdk-* repositories to be triggered so that they get the latest package updates, because it's actually the git-sdk-* files that are used to build the installer/PortableGit/etc.

In the same spirit as git-for-windows/gfw-helper-github-app#40 verifies that release notes about new packages are only added once the respective packages have actually been deployed, let's also add a sanity check to /git-artifacts to compare the timestamp of the git-sdk-* HEAD commit with the one from the respective Pacman index file, and refuse to run if the latter is newer (because that indicates that a sync is still needed to update the former).