Fix hg amend during arc diff

Summary:
When doing an arc diff with pending changes in your working copy
it was creating a new commit with the pending changes instead of amending
the existing one. The problem was the author comparison was comparing
values like "John Smith <john@foo.com>" with "John Smith". The fix changes
$api->getAuthor() to return "John Smith" instead of the full string. This
matches the behavior (and implementation) found in the git api.

Test Plan:
hg book foo
touch a && hg commit -Ama
touch b && hg add b
arc diff
When prompted, amend the pending changes to the existing commit.

Verified that the changes were amended instead of making a new commit.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5706