Use the new Ref/Hardpoint infrastructure from the experimental branch to look up the revision associated with the base commit when you run arc diff. If it exists and there's exactly one open revision you authored associated with it, add "Depends on ..." to the summary. This is a bit rough but the whole experimental branch is a bit of a wilderness for now.
Also remove --cache (passthru flag for arc lint --cache, which I removed in D18643) and getUnderlyingWorkingCopyRevision() (no callsites).
(This won't yet work in Mercurial, and does not make sense in SVN.)