I want "fast forward my workdir to upstream if it's safe"

For the past few years, within any given 12 hour period, at least one person on #git would come up with the use case with which I'm really familiar (having needed it many times myself):

  I have a workdir tracking a remote.
  I may or may not have made local edits... I can't remember.
  I may or may not have made local commits... I can't remember.
  But the upstream might now have changed... please update me *safely*.

Seriously, this comes up, a lot. Typically, from a cron job updater, but often just "what can I do to make sure I'm up to date".

So, what I need is a command, likely an option to "git merge" that says "do everything that a git merge would do except abort if it would have been a merge commit". In other words, abort if the workdir is dirty or is not a fast-forward update to the upstream. Bonus if it exits non-zero if something went wrong.

Please don't tell me "use these three commands in this script". I want a *command* I can tell people in #git.