Arcanist - make Patch workflow automagically apply dependencies

Summary: Nice title. Ref T479.

Test Plan:
Actually, help on that? I want to make sure I properly build up the "depends" on data. Is it as simple as

  -- observe at some commit hash RAZZMATAZZ
  -- git checkout -B "foo"
  -- <work>
  -- git commit -m "stash"
  -- arc diff -> yields DX
  -- git checkout -B "foo_prime"
  -- <work>
  -- git commit -m "stash"
  -- arc diff -> yield DY
  -- git checkout RAZZMATAZZ
  -- arc patch DY
  -- get prompted in workflow, agree
  -- git log and observe DX and DY applied

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran, davidressman

Maniphest Tasks: T479

Differential Revision: https://secure.phabricator.com/D6790