Write a git alias for fast forward that works in any tracking branch?

Kudos to the other two answers, but there's an easier way, assuming that your branches are tracking the remote branches:

git pull --ff-only

Bonus: this will work even if your branches are tracking branches in a repo other than origin, or tracking differently-named branches. Also it's really short.

I think this could work:

[alias]
    ff = !sh -c 'branch=$(git symbolic-ref HEAD | cut -d '/' -f 3) && git merge --ff-only origin/$branch' -