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