Record commit email information from `arc`

Summary:
Record author email information in `arc diff`, so we can recreate it in `arc patch` and elsewhere without creating any kind of email exposure issues.

In Mercurial, we currently store the whole string ("username <email@domain.com>"). Make this consistent with Git.

Test Plan: Created git and hg diffs, saw authorEmail populated.

Reviewers: btrahan, edward, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4825