Ref T2543. This updates the UI control in the web UI. Also:
- This implicitly makes this queryable with the API (differential.revision.search); it previously was not.
- This does NOT migrate existing saved queries. I'll do those in the next change, and hold this until it happens.
- This will break some existing /differential/?status=XYZ links. For example, status=open now needs to be status=open(). I couldn't find any of these in the upstream, and I suspect these are rare in the wild (users would normally link directly to saved queries, not use URI query construction).
