Allow to push revisions to review by himself to Differential (bug T1879, change 1/2, Differential part)

Summary: Checks if the revision author is in reviewers only if differential.allow-self-accept is false.

Test Plan:
Tested locally pushing revisions with "arc diff" to a Phabricator
server with differential.allow-self-accept at true or false with
myself or not as reviewer.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1879

Differential Revision: https://secure.phabricator.com/D3673