Ref T4195. A legitimate rule which needs this field is "do not allow commits as root". Interestingly, we have exactly one commit as root in each Phabricator, Arcanist and libphutil.

Since the committer and author don't need to be Phabricator accounts (just the Pusher), the existing "Committer" and "Author" fields can't express this rule (they'll be empty).

{F93406}