Drive-by comment: we need to document this new event up at the top of the module.  Or it might be simpler to just send a pair of `"workingSetAdd"` + `"workingSetRemove"` events instead... it looks like the existing code would do the right thing already, and then we wouldn't need the new _handleFileReplace() handler.  (And it would avoid breaking compatibility with any extensions that are trying to track the state of the working set).