In my opinion, "commit" should fail early and clearly. "Update" should do an automatic revert (with a printed line notifying the fact), for all files that have not been "rm'ed" from fossil. This is nothing more than saying that the merge algorithms do the following:

a) 2 ascii files: automatic merge

b) 2 binary files: error

c) 1 file in fossil and nothing in user checkout: automatic revert