Looking at the code now, I was unable to find a good reason as why we only rely on mergestate extras info after checking whether a filelog parent is ancestor of other or not.
I mean if we have stored in mergestate that
other was chosed, we should
blindly pick that one.
This cleanup will also help introduce more cases when both
fparent2 are non-null but using info from mergestate, we can fastpath.