When arc pushes to the staging area, tell Phabricator what we did

Summary:
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!!~").

Test Plan:
Ran arc diff a few times, then looked in the database for properties.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10093

Differential Revision: https://secure.phabricator.com/D15426