In "arc land", rebase branches in natural order

Summary: Ref T13546. When "arc land" performs cascading rebases, do them in "feature1", "feature2", etc., order so they're easier to follow. The outcome is not dependent on execution order.

Test Plan: Landed a change which cascaded many other branches, saw more comprehensible update order.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21320