Use hg phases to detect outgoing when phases are supported

Summary:
There was an accidental ! in the phase vs outgoing condition
which caused it to use 'hg outgoing' when it should have used the draft()
phase. Fixing this shaves 4.5 seconds off 'arc diff' on large repos.

Test Plan:
Ran arc diff --trace. Noted that the draft() was used and that the diff
contained the correct files and commit.

Reviewers: epriestley

Reviewed By: epriestley

CC: sid0, bos, dschleimer, aran, Korvin

Differential Revision: https://secure.phabricator.com/D5182