Don't parse mercurial usernames without email address as an email address

Summary:
This leads to information being lost when others do `arc patch` because the name is used as the email address.

For example:
  username = Richard van Velzen

Would give:
  'authorName' => null,
  'authorEmail' => 'Richard van Velzen'

Test Plan:
ran it through my head a couple of times, and tested it with the common options which all gave the expected result:
  'rvanvelzen@company.com',
  'Richard van Velzen',
  'Richard van Velzen <rvanvelzen@company.com>',
  'Richard van Velzen rvanvelzen@company.com',

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9605