Currently, we record the branch name of a diff when "arc diff" is run, and show it in Differential like:
Branch: examplebranch
It would be nice to also record the most recent remote-tracking branch name in history, if it's available (maybe with some reasonable cutoff for performance, so we only examine the N most recent commits to try to find a remote-tracking branch). When a remote-tracking branch is found, display something like this:
Branch: examplebranch (branched from master)