fixed retrieving origin-SHA for check-repo-in-sync task

I have found the bug from #1. The origin-SHA was retrieved wrong. Jenkins uses this command

```
git rev-parse remotes/origin/master^{commit}
```

to retrieve the commit SHA of the origin, so I added this here, too.

And: Watch out if you create a PR. Sometimes you have to click on that unremarkable **Edit** button on the top right corner (gray on gray button) to fix where the PR is targeted to ;-)