Add 'arc which', and ArcanistRepositoryAPI->loadWorkingCopyDifferentialRevisions()

Summary:

• See T787.
• @cpiro has an immediate use case for this, which is ##arc amend --revision

arc which --id --show master## for "git merge --autosquash" or similar.

• For T614, we need this to choose "--create" vs "--update".
• Other workflows should also use this to improve how often we automatically

get things right, particularly in Mercurial and SVN.

Test Plan:
Ran "arc which" in SVN, Git and HG working copies with various flags;
results seemed reasonable.

Reviewers: btrahan, cpiro, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T787

Differential Revision: https://secure.phabricator.com/D1478