Depends on D18651. Allows Mercurial to auto-detect depends-on revisions.
See D18652 and D18653. Didn't actually test the empty repository states but they're probably okay. The experimental branch is an untamed wild. ¯\_(ツ)_/¯
For the initial arc diff it seemed to work. I got a commit message pre-populated with Depends On: Dxxxx.
I ran into some strange PhutilConsole import errors on second update but when I came here to report they become non-reproducible so I'll keep my eye open for them but no other issues so far.