When arc pushes to the staging area, tell Phabricator what we did
Ref T10093. Right now, Phabricator kind of guesses that arc probably pushed stuff to the staging area.
This can cause confusing/misleading errors later, if it didn't actually push.
Instead, tell Phabricator that we pushed, so we can raise more tailored messages in the web UI (e.g., make "Land Revision" say "this wasn't pushed to the staging area" instead of "whoops, error!!~").
Ran arc diff a few times, then looked in the database for properties.
Reviewed By: chad
Maniphest Tasks: T10093
Differential Revision: https://secure.phabricator.com/D15426