Parse Conflicts field in commit message

Summary:
Git adds it automatically.

I don't like this solution much because there could be other unknown fields appended to the end of the commit message which will break parsing.

Test Plan:
Reparsed commit ending with:

Differential Revision: ...
Conflicts: ...

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4157