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