Refactor author handling in the Mercurial API

Summary: There were two callsites which needed some information from the username. Only one worked "correctly", causing arc diff to not amend commits anymore because the author could not be parser.

Test Plan: run arc diff with changes

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9797