Some improvements to git-workflow scripts

* `wip` calls `git fetch`, so we don’t end up branching from an out of
  date upstream and
* `ready` does the right thing now when re-running after a merge
  conflict or failed tests when there was a merge commit (it used to try
  squashing because it saw multiple commits on the ready branch, but now
  we filter the merge commits, so it sees only one).