It should be also valid to have empty change under hg

Summary: Like git, it is also valid to have empty net change under hg. Should not throw if that is the case.

Test Plan: Tried on a hg repo.

Reviewers: lifeihuang, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8888