If we want to push only one branch, we'll have to specify branch and
remote explicitly. Pushing to origin doesn't work for everyone, since
some of us have a origin remote that can't be pushed to. Using plain
`git push` has the problem that it'll try pushing all checked out
branchs which fails e.g. if some branches (staging, staging-next, …) are
behind their remote counterparts.
The solution is to require everyone to configure a per branch pushRemote
for haskell-updates which will then be used by merge-and-open-pr.sh.
