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