Provide a smoother "update diff" web workflow

Summary:
Fixes T1102. If you don't use arc, the web workflow requires some extra needless steps when updating diffs.

Provide a more streamlined "Update Diff" workflow.

Test Plan: {F347750}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1102

Differential Revision: https://secure.phabricator.com/D12131