Hm... the way we our systems folks have it installed it's apparently not a git repository. However, I was able to look in the src directory and found those lines in workflow/ArcanistCommitWorkflow.php. I'm checking with the person who ran the command that prompted me to make this feature request.

Oh! So it already has this feature! I wonder if perhaps we are on an old version. I tried arc version as specified here, but I just get an error: