Ok, now I understand. Yes, git expects you to commit or stash before doing that, matching the philosophy that there should be a commit you can go back to undo (even if you have to look at the reflog to see what that commit is).

Personally, I commit and stash all the time, and feel better than the old CVS in which I manually stashed (by copying) the pre-"cvs update" state in case I needed to go back to it.

matsushiko above mentions that git now has --autostash; legit had "sync" which does stash-rebase-unstash - but I rarely ever needed that workflow.