I don't use git a lot, so I am not all that familiar with it's work flow. But I think fossil assumes a change will go into the commit so you use the stash command to hide items you don't want in. while git assumes a change does not go in a commit so you use add on files that you want in.

A bigger question is what to do when you only want specific chunks from the diff in your commit. I tend to faff around with stash, interactive sdiff and hand editing the patch when I need to dissect a chunk, a situation I feel could be better.