In "arc patch", update submodules slightly later
I believe it is safe to just update them a little later, after the intermediate branch management logic runs.
Test Plan: Ran arc patch --nobranch, saw submodule update run later. Not 100% sure this doesn't cause weird issues, but I can't anticipate any.
Reviewers: amckinley, jmeador
Reviewed By: jmeador
Maniphest Tasks: T13151
Differential Revision: https://secure.phabricator.com/D19475