Tag relevant parties in merge conflicts

[RikkiGibson]

I think a pretty decent heuristic for automatically identifying the person to contact in case of a merge conflict is to get the "most recent author" from both the base and head versions of the conflicting file, and simply tag them in the PR. Sometimes we will tag the wrong person, but most of the time it will be right, and it won't require anything complicated like obtaining the conflicting regions and running a blame on them.