Support short version of arc diff --add-all

Summary: Like `git commit -a`.

Test Plan:
  $ arc diff -a

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4903
Support short version of arc diff --add-all

Summary: Like `git commit -a`.

Test Plan:
  $ arc diff -a

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4903