Always merge with `ff = only`

@jferris:

This has the effect that Git will never implicitly do a merge commit,
including while doing git pull. If I'm unexpectedly out of sync when I
git pull, I get this message:

> fatal: Not possible to fast-forward, aborting.

At that point, I know that I've forgotten to do something in my normal
workflow, and I retrace my steps.

@gylaz:

I like the value of seeing a failure message if a pull (or just merge)
fails due to not being able to get fast-forwarded. Then I can look to
see whose changes went out, that I may have not expected to be there.