phabricator: update diff property even if we choose not to create a new diff The diff property contains metadata like "HG Node". Previously we skip uploading a new diff if we are sure that the old patch and new patch have a same content. That has issues when a pusher adds an obsmarker using the old "HG Node" stored in the old diff. This patch adds logic to update the diff property so "HG Node" gets updated to prevent that issue. Differential Revision: https://phab.mercurial-scm.org/D229