Return the changeset ID as part of the changeset dict

Summary:
getDiffDict method returns the JSON for the changesets in a diff
The JSON descriping a changeset didn't containt the changeset ID
in some scenarios, knowing the changeset ID is really useful for the
clients.(i.e. when you want to open a standalone view of the changeset,
    you need to know its ID)

Test Plan: existing

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5816