This patch allows merging of non-conflicting changes outside narrowspec.
I'm pretty sure this doesn't actually perform the merge, it just drops the changes outside the narrowspec. On commit, we need to record that outside/ has the new nodeid that we got from the side we merged in. To do that, we need to remember what that nodeid is, from the time of hg merge to the time of hg commit. That probably means storing the nodeid in the dirstate (like git does), or maybe in the merge state.
I think neither of them currently contains files outside narrows, so we'd need to extend whichever we decide. It's probably easier to extend the merge state. I'm not sure which I think is cleaner to extend. There's precedent for populating the commit based on the dirstate (index) in git.