Make "arc diff" sort of detect dependent revisions

Summary:
Depends on D18645. Fixes T11343. Currently, when you "arc diff", we don't detect dependencies even if your working copy is on top of another open revision you authored.

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.)

Test Plan: Created this revision, got an auto "depends on" up top there.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11343

Differential Revision: https://secure.phabricator.com/D18651